Truncation strategy for degenerate subspaces

[mtfishman]

To-do:

• Add tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.63%. Comparing base (f166200) to head (73ac13a).
Report is 1 commits behind head on main.

Additional details and impacted files

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+ Coverage   94.41%   94.63%   +0.22%     
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  Files          14       14              
  Lines         412      429      +17     
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+ Hits          389      406      +17     
  Misses         23       23              

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[mtfishman]

@lkdvos curious to hear your thoughts on this design. There are clearly many ways to numerically detect degenerate subspaces, the current design checks if neighboring values in the spectrum are close to each other up to the specified tolerances.

[lkdvos]

Except for some style things looks good to me!