[Containers] fix bug in DenseAxisArrayView

[odow]

Closes #3175. I overlooked the part where taking a view drops the dimension if it contains a single element.

[codecov]

Codecov Report

Base: 98.07% // Head: 97.42% // Decreases project coverage by -0.64% ⚠️

Coverage data is based on head (0b5ae50) compared to base (1024661).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3180      +/-   ##
==========================================
- Coverage   98.07%   97.42%   -0.65%     
==========================================
  Files          33       33              
  Lines        4563     4115     -448     
==========================================
- Hits         4475     4009     -466     
- Misses         88      106      +18     

Impacted Files	Coverage Δ
src/Containers/DenseAxisArray.jl	93.77% <100.00%> (-2.15%)	⬇️
src/precompile.jl	0.00% <0.00%> (-100.00%)	⬇️
src/Containers/Containers.jl	88.88% <0.00%> (-2.78%)	⬇️
src/mutable_arithmetics.jl	86.20% <0.00%> (-2.15%)	⬇️
src/operators.jl	94.58% <0.00%> (-2.06%)	⬇️
src/Containers/SparseAxisArray.jl	97.36% <0.00%> (-1.07%)	⬇️
src/JuMP.jl	95.35% <0.00%> (-0.75%)	⬇️
src/aff_expr.jl	96.69% <0.00%> (-0.56%)	⬇️
src/constraints.jl	96.15% <0.00%> (-0.35%)	⬇️
... and 21 more

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[odow]

@metab0t do you want to test this branch out?

[metab0t]

x-ref: #3175 (comment)

[odow]

Try now

[metab0t]

The dimension of view is correct now.

[odow]

This whole notion of a view into a dense axis array is messy, because view of normal arrays assume everything is 1-indexed. I probably missed a few other methods. It's a shame Julia doesn't have formal interfaces for subtypes.

[metab0t]

Implementing a full-featured custom array type (to act like internal Array) in Julia requires a lot of work due to the expansion of API.
Anyway, having proper support of view for DenseAxisArray is great!