using `mean` with distributed Dagger arrays

It would be great to have functions from `Statistics` useable with distributed Dagger arrays. Currenly the function `sum` does work but the equivalent `mean` function produces the error below:

```julia
using Dagger
using Statistics

sz = (2,2,3)
X = randn(sz)
DX = Distribute(Blocks(sz[1],sz[2],1), X)

# works
@show sum(DX,dims=3)
# fails
@show mean(DX,dims=3)
```
I am using Dagger v0.13.1 in Julia v1.6.2.

By looking at the code of `Statistics.jl` it seems that a specific implementation of
`mapreduce(f,g,d::Dagger.Distribute,dims=dims) ` would be needed. There is already one without the `dims` parameter. Would a PR be helpful?

Full stack-trace:

```
ERROR: LoadError: MethodError: no method matching +(::Float64, ::Dagger.GetIndex{Float64, 3})
For element-wise addition, use broadcasting with dot syntax: scalar .+ array
Closest candidates are:
+(::Any, ::Any, ::Any, ::Any...) at operators.jl:560
+(::Union{Float16, Float32, Float64}, ::BigFloat) at mpfr.jl:392
+(::AbstractFloat, ::Bool) at bool.jl:102
...
Stacktrace:
[1] add_sum(x::Float64, y::Dagger.GetIndex{Float64, 3})
@ Base ./reduce.jl:24
[2] macro expansion
@ ./reducedim.jl:270 [inlined]
[3] macro expansion
@ ./simdloop.jl:77 [inlined]
[4] _mapreducedim!(f::Statistics.var"#4#6"{typeof(identity), Dagger.BCast{Base.Broadcast.Broadcasted{Dagger.DaggerBroadcastStyle, Tuple{Base.OneTo{Int64}}, typeof(/), Tuple{Dagger.GetIndex{Float64, 3}, Int64}}, Float64, 1}}, op::typeof(Base.add_sum), R::Array{Float64, 3}, A::Distribute{Float64, 3})
@ Base ./reducedim.jl:269
[5] mapreducedim!(f::Function, op::Function, R::Array{Float64, 3}, A::Distribute{Float64, 3})
@ Base ./reducedim.jl:277
[6] _mapreduce_dim(f::Function, op::Function, #unused#::Base._InitialValue, A::Distribute{Float64, 3}, dims::Int64)
@ Base ./reducedim.jl:324
[7] #mapreduce#672
@ ./reducedim.jl:310 [inlined]
[8] #_sum#702
@ ./reducedim.jl:900 [inlined]
[9] _sum
@ ./reducedim.jl:900 [inlined]
[10] #sum#680
@ ./reducedim.jl:874 [inlined]
[11] _mean(f::typeof(identity), A::Distribute{Float64, 3}, dims::Int64)
@ Statistics /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Statistics/src/Statistics.jl:177
[12] #mean#2
@ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Statistics/src/Statistics.jl:164 [inlined]
[13] top-level scope
@ show.jl:955
in expression starting at /home/abarth/.julia/dev/NCDatasets/test/test_dagger.jl:46
```