implemented Base.unaliascopy for mappedarrays

[m-wells]

I was getting this error

ERROR: ArgumentError: an array of type `MappedArrays.MappedArray` shares memory with another argument and must
make a preventative copy of itself in order to maintain consistent semantics,
but `copy(A)` returns a new array of type `Array{Unitful.Quantity{Float32,𝐌,Unitful.FreeUnits{(M⊙,),𝐌,nothing}},1}`. To fix, implement:
    `Base.unaliascopy(A::MappedArrays.MappedArray)::typeof(A)`

So I did what the error message suggested.

[codecov-io]

Codecov Report

Merging #30 into master will increase coverage by 0.02%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   74.44%   74.46%   +0.02%     
==========================================
  Files           1        1              
  Lines          90       94       +4     
==========================================
+ Hits           67       70       +3     
- Misses         23       24       +1

Impacted Files	Coverage Δ
src/MappedArrays.jl	74.46% <75%> (+0.02%)	⬆️

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[timholy]

Can you show how this came up? It's quite possible that this error is a good thing, and that you should instead be explicitly calling copy.

[m-wells]

I have a large memory mapped array with JLD that I call views and mapped arrays on. I tried to create a standalone example that would demonstrate the error but I cannot. The views and mapped arrays all share the underlying data.

To be clear, the underlying data is not being manipulated during these operations and the views and maps are being passed to functions to compute new values to be stored separately.

[timholy]

Even if you can't share the data for reasons of size, can you share the code portion of a reproducer?

[m-wells]

I'll see if I can make a code snippet that I am comfortable sharing.