DM-33326: Fix round-tripping of spectractor spectra

[mfisherlevine]

Checklist

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

[codecov]

Codecov Report

Merging #644 (3cc6914) into main (5f8210b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #644   +/-   ##
=======================================
  Coverage   83.78%   83.78%           
=======================================
  Files         239      239           
  Lines       30432    30432           
  Branches     5055     5055           
=======================================
  Hits        25498    25498           
  Misses       3789     3789           
  Partials     1145     1145           

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[czwa]

Can't this just be a Spectraction?

Don't forget to add the change message to the doc directory.