Broadcasting weirdness

Hi, thanks for this great package!

I'm seeing some strange behavior with broadcasting. It's easy enough to work around, but still seems worth noting:
```julia
julia> (zeros(3) .± 1) .± 2
3-element Array{Measurement{Measurement{Float64}},1}:
0.0 ± 1.0 ± 2.0 ± 0.0
0.0 ± 1.0 ± 2.0 ± 0.0
0.0 ± 1.0 ± 2.0 ± 0.0
```

No source files or tests are named. Reproduce the Julia expression `(zeros(3) .± 1) .± 2` and inspect the package's broadcasting behavior; completion would require resolving the reported nested-measurement result, with the expected output to be established from the issue discussion.

Written by the indexing model from the issue text.