add regularize_spectra() function

[ntessore]

Adds the regularize_spectra() function to regularise a set of angular power spectra, i.e. change its associated covariance matrix to be positive definite.

[codecov-commenter]

Codecov Report

Attention: 55 lines in your changes are missing coverage. Please review.

Comparison is base (2f14bf0) 100.00% compared to head (90b81ac) 52.58%.

Files	Patch %	Lines
src/angst/_spectra.py	14.06%	55 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##              main       #2       +/-   ##
============================================
- Coverage   100.00%   52.58%   -47.42%     
============================================
  Files            4        4               
  Lines           54      116       +62     
============================================
+ Hits            54       61        +7     
- Misses           0       55       +55     

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