[Containers] maintenance of SparseAxisArray.jl

[odow]

In particular, this cleans up the broadcast implementation to avoid
private Base functions.

I started to look at the size issue, but I got distracted by style issues. Then I saw the broadcast implementation called Base.Broadcast._broadcast_getindex(bc, I) so I fixed that. I can break up the style part if requested.

[codecov]

Codecov Report

Merging #2762 (2678761) into master (6b48409) will increase coverage by 0.05%.
The diff coverage is 96.66%.

@@            Coverage Diff             @@
##           master    #2762      +/-   ##
==========================================
+ Coverage   94.18%   94.23%   +0.05%     
==========================================
  Files          43       43              
  Lines        5520     5517       -3     
==========================================
  Hits         5199     5199              
+ Misses        321      318       -3     

Impacted Files	Coverage Δ
src/Containers/SparseAxisArray.jl	96.38% <96.66%> (+3.36%)	⬆️

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[blegat]

What's the reason you define copy instead of copyto! ?

[odow]

copyto! needs to precompute the axes and call similar. That's a win for things like arrays where you can precompute the storage. For dictionaries, it's less obvious. For example, we weren't preallocating the size of the dictionary in similar.

In addition, if we defined copyto! we have to implement some of the weirder broadcast methods like extrude and newindex to get everything to work. If we implement copy, it's a lot simpler.