Define isstored for SubArray

[mtfishman]

This generalizes a definition originally made in ITensor/BlockSparseArrays.jl#74, with the goal of fixing ITensor/BlockSparseArrays.jl#73.

@lkdvos ideally this code would handle things like linear indexing but that system is being rewritten in #39 so for now I'm just following the style of how other isstored functions are defined.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.49%. Comparing base (d741b7f) to head (5d45797).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
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+ Coverage   71.36%   71.49%   +0.13%     
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  Files           7        7              
  Lines         433      435       +2     
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+ Hits          309      311       +2     
  Misses        124      124              

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docs	37.26% <0.00%> (-0.18%)	⬇️

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[mtfishman]

I missed that I actually had made basically the same definition of isstored for SubArray, but it was restricted to sparse arrays, so I just defined a similar definition for all arrays. @lkdvos let me know what you think (I'm leaving the sparse array definition since it is in an eval loop anyway).

Probably we can make more of the sparse array definitions for wrappers more general, but I'd prefer to leave that for a future PR since I think that is more involved.