add FITS saving and loading

[jmbhughes]

Add FITS saving and loading

Closes #222

[codecov]

Codecov Report

Attention: Patch coverage is 94.02985% with 4 lines in your changes missing coverage. Please review.

Project coverage is 87.78%. Comparing base (15876d9) to head (388d947).
Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
regularizepsf/psf.py	93.10%	2 Missing ⚠️
regularizepsf/transform.py	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #224      +/-   ##
==========================================
+ Coverage   87.55%   87.78%   +0.22%     
==========================================
  Files          12       12              
  Lines         836      884      +48     
==========================================
+ Hits          732      776      +44     
- Misses        104      108       +4     

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