[Containers] add support for view of DenseAxisArray

[odow]

Closes #3151
Part of #2998 (still to do SparseAxisArray?)

I'm not overly convinced of this approach. I felt I was just poking methods until things were working. For something like D[I] .= 1.0, Base uses a weird dispatch down dotview and maybeview.

I should add more tests before merging, and consider also SparseAxisArray.

There's also a question: should you be able to access a value that isn't in the view, but is in the underlying array?

[codecov]

Codecov Report

Base: 97.70% // Head: 97.72% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (3856b82) compared to base (ed3dc0f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3152      +/-   ##
==========================================
+ Coverage   97.70%   97.72%   +0.01%     
==========================================
  Files          33       33              
  Lines        4485     4522      +37     
==========================================
+ Hits         4382     4419      +37     
  Misses        103      103              

Impacted Files	Coverage Δ
src/Containers/DenseAxisArray.jl	95.86% <100.00%> (+0.66%)	⬆️

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[odow]

Okay. I fixed the getindex problem to respect the new axes, changed eachindex to be lazy, and added a couple more tests.