[NDTensors] BlockSparseArray contract, QR, and Hermitian eigendecomposition

[mtfishman]

This defines some more tensor and matrix algebra for BlockSparseArray, such as contraction, QR, and Hermitian eigendecomposition.

[emstoudenmire]

I see what you mean about the decompositions. The QR code here is very compact – much of it by line number seems to mainly be the various small helper functions, then the core code actually doing the algorithm is quite short.

[codecov-commenter]

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Comparison is base (9060282) 85.44% compared to head (9ae8885) 54.80%.

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- Coverage   85.44%   54.80%   -30.64%     
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  Files          89       88        -1     
  Lines        8402     8349       -53     
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- Hits         7179     4576     -2603     
- Misses       1223     3773     +2550     

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