[NDTensors] Add DiagonalArrays submodule

[mtfishman]

Description

Adds a DiagonalArrays submodule, which defines a type DiagonalArray which generalizes LinearAlgebra.Diagonal to n-dimensional arrays.

This follows up on efforts in #1206, #1205, #1222, and #1213 to replace the TensorStorage subtypes Dense, Diag, BlockSparse, and DiagBlockSparse with Julia AbstractArrays. So far this just implemented basic AbstractArray interface functions but not the functions needed for it to be used as an ITensor backend, that will be left for future work.

@kmp5VT @emstoudenmire

This is branched off of #1222 so I won't merge this until that is merged.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (690b219) 85.37% compared to head (78a0bad) 67.21%.

❗ Current head 78a0bad differs from pull request most recent head 9c457d9. Consider uploading reports for the commit 9c457d9 to get more accurate results

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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1225       +/-   ##
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- Coverage   85.37%   67.21%   -18.17%     
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  Files          89       88        -1     
  Lines        8453     8415       -38     
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- Hits         7217     5656     -1561     
- Misses       1236     2759     +1523     

see 40 files with indirect coverage changes

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