Block sparse eig

[mtfishman]

Also simplifies and generalizes the default_f_algorithm logic across factorizations. Depends on QuantumKitHub/MatrixAlgebraKit.jl#32.

To do:

• Add tests.
• Check the inputs are block diagonal and square.
• eig[h]_trunc.
• eig[h]_vals.
• Make the type of the block diagonal matrix storing the eigenvalues more specific. In particular, right now the blocks are dense, while they should be Diagonal. To implement that, I plan to add support for similar(A, BlockType(...)) as discussed elsewhere.

[codecov]

Codecov Report

Attention: Patch coverage is 96.87500% with 2 lines in your changes missing coverage. Please review.

Project coverage is 74.84%. Comparing base (ea250bb) to head (4df87f4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/factorizations/eig.jl	94.73%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
+ Coverage   74.26%   74.84%   +0.57%     
==========================================
  Files          34       35       +1     
  Lines        1702     1749      +47     
==========================================
+ Hits         1264     1309      +45     
- Misses        438      440       +2     

Flag	Coverage Δ
docs	6.52% <0.00%> (-0.18%)	⬇️

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