Ensure arithmetic results preserve spectral axis shifts

[rosteen]

Fixes #1131. We outsource most of the arithmetic logic to the inherited upstream handling, but it seems that wasn't preserving the outcome of shift_spectrum_to for some reason. I don't fully understand how the results were ending up with the original spectral axis, since that method updates self._spectral_axis, but this workaround is sufficient to give the expected behavior for now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.88%. Comparing base (d16d91a) to head (d2938eb).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1158      +/-   ##
==========================================
+ Coverage   86.83%   86.88%   +0.04%     
==========================================
  Files          63       63              
  Lines        4528     4544      +16     
==========================================
+ Hits         3932     3948      +16     
  Misses        596      596              

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