Tests failing

Started on #47 but having problem getting the current release to pass tests:

```julia
Testing Running tests...
Test Summary: | Pass Total
types | 15 15
┌ Warning: Error parsing "'ve". AssertionError("No parser for \"'ve\"")
└ @ CorpusLoaders ~/.julia/packages/CorpusLoaders/RkzqR/src/apply_subparsers.jl:23
Test Summary: | Pass Total
SemCor | 35 35
Test Summary: | Pass Total
Senseval3 | 4 4
Test Summary: | Pass Total
CoNLL | 9 9
Test Summary: | Pass Total
CoNLL2000 | 7 7
basic use train_pos: Test Failed at /Users/anthony/.julia/packages/CorpusLoaders/RkzqR/test/test_IMDB.jl:15
Expression: typeof(words) == Vector{String}
Evaluated: Vector{Any} == Vector{String}
```

```
Stacktrace:
[1] macro expansion
@ ~/.julia/packages/CorpusLoaders/RkzqR/test/test_IMDB.jl:15 [inlined]
[2] top-level scope
@ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1226
[3] include(fname::String)
@ Base.MainInclude ./client.jl:444
[4] macro expansion
@ ~/.julia/packages/CorpusLoaders/RkzqR/test/runtests.jl:20 [inlined]
[5] top-level scope
@ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1226
[6] include(fname::String)
@ Base.MainInclude ./client.jl:444
[7] top-level scope
@ none:6
[8] eval
@ ./boot.jl:360 [inlined]
[9] exec_options(opts::Base.JLOptions)
@ Base ./client.jl:261
[10] _start()
@ Base ./client.jl:485
basic use train_pos: Test Failed at /Users/anthony/.julia/packages/CorpusLoaders/RkzqR/test/test_IMDB.jl:18
Expression: typeof(sentences) == #= /Users/anthony/.julia/packages/CorpusLoaders/RkzqR/test/test_IMDB.jl:18 =# @NestedVector(String, 2)
Evaluated: Vector{Any} == Vector{Vector{String}}
Stacktrace:
[1] macro expansion
@ ~/.julia/packages/CorpusLoaders/RkzqR/test/test_IMDB.jl:18 [inlined]
[2] top-level scope
@ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1226
[3] include(fname::String)
@ Base.MainInclude ./client.jl:444
[4] macro expansion
@ ~/.julia/packages/CorpusLoaders/RkzqR/test/runtests.jl:20 [inlined]
[5] top-level scope
@ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1226
[6] include(fname::String)
@ Base.MainInclude ./client.jl:444
[7] top-level scope
@ none:6
[8] eval
@ ./boot.jl:360 [inlined]
[9] exec_options(opts::Base.JLOptions)
@ Base ./client.jl:261
[10] _start()
@ Base ./client.jl:485
basic use train_pos: Error During Test at /Users/anthony/.julia/packages/CorpusLoaders/RkzqR/test/test_IMDB.jl:19
Test threw exception
Expression: sum(length.(sentences)) == length(words)
MethodError: no method matching zero(::Type{Any})
Closest candidates are:
zero(::Type{Union{Missing, T}}) where T at missing.jl:105
zero(::Union{Type{P}, P}) where P<:Dates.Period at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Dates/src/periods.jl:53
zero(::SparseArrays.AbstractSparseArray) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/SparseArrays/src/SparseArrays.jl:55
...
Stacktrace:
[1] zero(#unused#::Type{Any})
@ Base ./missing.jl:106
[2] reduce_empty(#unused#::typeof(+), #unused#::Type{Any})
@ Base ./reduce.jl:311
[3] reduce_empty(#unused#::typeof(Base.add_sum), #unused#::Type{Any})
@ Base ./reduce.jl:320
[4] mapreduce_empty(#unused#::typeof(identity), op::Function, T::Type)
@ Base ./reduce.jl:343
[5] reduce_empty(op::Base.MappingRF{typeof(identity), typeof(Base.add_sum)}, #unused#::Type{Any})
@ Base ./reduce.jl:329
[6] reduce_empty_iter
@ ./reduce.jl:355 [inlined]
[7] mapreduce_empty_iter(f::Function, op::Function, itr::Vector{Any}, ItrEltype::Base.HasEltype)
@ Base ./reduce.jl:351
[8] _mapreduce(f::typeof(identity), op::typeof(Base.add_sum), #unused#::IndexLinear, A::Vector{Any})
@ Base ./reduce.jl:400
[9] _mapreduce_dim
@ ./reducedim.jl:318 [inlined]
[10] #mapreduce#672
@ ./reducedim.jl:310 [inlined]
[11] mapreduce
@ ./reducedim.jl:310 [inlined]
[12] #_sum#682
@ ./reducedim.jl:878 [inlined]
[13] _sum
@ ./reducedim.jl:878 [inlined]
[14] #_sum#681
@ ./reducedim.jl:877 [inlined]
[15] _sum
@ ./reducedim.jl:877 [inlined]
[16] #sum#679
@ ./reducedim.jl:873 [inlined]
[17] sum(a::Vector{Any})
@ Base ./reducedim.jl:873
[18] macro expansion
@ ~/.julia/packages/CorpusLoaders/RkzqR/test/test_IMDB.jl:19 [inlined]
[19] top-level scope
@ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1226
[20] include(fname::String)
@ Base.MainInclude ./client.jl:444
[21] macro expansion
@ ~/.julia/packages/CorpusLoaders/RkzqR/test/runtests.jl:20 [inlined]
[22] top-level scope
@ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1226
[23] include(fname::String)
@ Base.MainInclude ./client.jl:444
[24] top-level scope
@ none:6
[25] eval
@ ./boot.jl:360 [inlined]
[26] exec_options(opts::Base.JLOptions)
@ Base ./client.jl:261
[27] _start()
@ Base ./client.jl:485
basic use train_pos: Test Failed at /Users/anthony/.julia/packages/CorpusLoaders/RkzqR/test/test_IMDB.jl:21
Expression: typeof(flatten_levels(docs, (!lvls)(IMDB, :documents)) |> full_consolidate) == Vector{String}
Evaluated: Vector{Any} == Vector{String}
Stacktrace:
[1] macro expansion
@ ~/.julia/packages/CorpusLoaders/RkzqR/test/test_IMDB.jl:21 [inlined]
[2] top-level scope
@ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1226
[3] include(fname::String)
@ Base.MainInclude ./client.jl:444
[4] macro expansion
@ ~/.julia/packages/CorpusLoaders/RkzqR/test/runtests.jl:20 [inlined]
[5] top-level scope
@ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1226
[6] include(fname::String)
@ Base.MainInclude ./client.jl:444
[7] top-level scope
@ none:6
[8] eval
@ ./boot.jl:360 [inlined]
[9] exec_options(opts::Base.JLOptions)
@ Base ./client.jl:261
[10] _start()
@ Base ./client.jl:485
basic use train_pos: Test Failed at /Users/anthony/.julia/packages/CorpusLoaders/RkzqR/test/test_IMDB.jl:22
Expression: typeof(flatten_levels(docs, (!lvls)(IMDB, :sentences)) |> full_consolidate) == Vector{String}
Evaluated: Vector{Any} == Vector{String}
Stacktrace:
[1] macro expansion
@ ~/.julia/packages/CorpusLoaders/RkzqR/test/test_IMDB.jl:22 [inlined]
[2] top-level scope
@ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1226
[3] include(fname::String)
@ Base.MainInclude ./client.jl:444
[4] macro expansion
@ ~/.julia/packages/CorpusLoaders/RkzqR/test/runtests.jl:20 [inlined]
[5] top-level scope
@ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1226
[6] include(fname::String)
@ Base.MainInclude ./client.jl:444
[7] top-level scope
@ none:6
[8] eval
@ ./boot.jl:360 [inlined]
[9] exec_options(opts::Base.JLOptions)
@ Base ./client.jl:261
[10] _start()
@ Base ./client.jl:485
Test Summary: | Pass Fail Error Total
IMDB | 19 4 1 24
basic use train_pos | 1 4 1 6
basic use train_neg | 6 6
basic use test_pos | 6 6
basic use test_neg | 6 6
Test Summary: | Pass Fail Error Total
IMDB | 19 4 1 24
basic use train_pos | 1 4 1 6
basic use train_neg | 6 6
basic use test_pos | 6 6
basic use test_neg | 6 6
ERROR: LoadError: Some tests did not pass: 19 passed, 4 failed, 1 errored, 0 broken.
in expression starting at /Users/anthony/.julia/packages/CorpusLoaders/RkzqR/test/runtests.jl:19

caused by: Some tests did not pass: 19 passed, 4 failed, 1 errored, 0 broken.
ERROR: Package CorpusLoaders errored during testing
```

```julia
(jl_4k2h6c) pkg> st
Status `/private/var/folders/4n/gvbmlhdc8xj973001s6vdyw00000gq/T/jl_4k2h6c/Project.toml`
[214a0ac2] CorpusLoaders v0.3.2

julia> versioninfo()
Julia Version 1.6.0
Commit f9720dc2eb (2021-03-24 12:55 UTC)
Platform Info:
OS: macOS (x86_64-apple-darwin19.6.0)
CPU: Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-11.0.1 (ORCJIT, skylake)
Environment:
JULIA_LTS_PATH = /Applications/Julia-1.0.app/Contents/Resources/julia/bin/julia
JULIA_PATH = /Applications/Julia-1.6.app/Contents/Resources/julia/bin/julia
JULIA_EGLOT_PATH = /Applications/Julia-1.5.app/Contents/Resources/julia/bin/julia
JULIA_NUM_THREADS = 5
JULIA_NIGHTLY_PATH = /Applications/Julia-1.7.app/Contents/Resources/julia/bin/julia
```

Start by running the failing tests in test/test_IMDB.jl through test/runtests.jl under Julia 1.6. Trace the IMDB train_pos path that produces Vector{Any} and compare it with the expected types shown in the failures. Done means the IMDB test group passes without the reported type errors or exception.

Written by the indexing model from the issue text.