Add variable PSF to variable PSF transforms

[jmbhughes]

PR summary

This allows a variable PSF to be the target.

Todos

None

Test plan

Unit tests pass. And works for creating PUNCH synthetic images.

Breaking changes

File format save is different now.

Related Issues

None

[codecov]

Codecov Report

Attention: Patch coverage is 71.81818% with 31 lines in your changes missing coverage. Please review.

Project coverage is 93.50%. Comparing base (cc937ef) to head (c40f2bb).
Report is 7 commits behind head on main.

Files	Patch %	Lines
regularizepsf/fitter.py	57.57%	14 Missing ⚠️
regularizepsf/psf.py	62.50%	9 Missing ⚠️
regularizepsf/corrector.py	89.36%	5 Missing ⚠️
regularizepsf/visualize.py	0.00%	2 Missing ⚠️
tests/test_corrector.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #203      +/-   ##
==========================================
- Coverage   95.19%   93.50%   -1.70%     
==========================================
  Files          10       10              
  Lines        1020     1062      +42     
==========================================
+ Hits          971      993      +22     
- Misses         49       69      +20     

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