Add filter to group key

[gbrammer]

Add blocking filter to key when combining spectra so that G140M-F070LP and G140M-F100LP aren't degenerate.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.

Project coverage is 67.20%. Comparing base (71dfe82) to head (5a9faf9).

Files with missing lines	Patch %	Lines
msaexp/slit_combine.py	54.54%	5 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #81   +/-   ##
=======================================
  Coverage   67.20%   67.20%           
=======================================
  Files          16       16           
  Lines        7151     7151           
=======================================
  Hits         4806     4806           
  Misses       2345     2345           

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