DM-30966: Make Spectractions Gen3-serializable

[mfisherlevine]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Merging #633 (69ef0ca) into main (19f9d62) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #633   +/-   ##
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  Coverage   84.13%   84.13%           
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  Files         237      237           
  Lines       30216    30216           
  Branches     5022     5022           
=======================================
  Hits        25423    25423           
  Misses       3648     3648           
  Partials     1145     1145           

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[czwa]

I think I'd be happier if these used the FitsGenericFormatter, but I think that would involve pushing changes into the spectractor code, and I'm not clear on how difficult that is. The Spectrum and Image types would need to implement readFits and writeFits methods (likely as wrappers around the existing load_X and save_X methods).