Generalize whitespace handling in splitting modification column FragPipe

[rodvrees]

Small improvement to the parsing logic for modifications in the _parse_peptidoform function in psm_utils/io/fragpipe.py. I am not sure of the reason why, but sometimes FragPipe output separates modification by ", " and sometimes just ",". Could have to do with versioning. This change ensures that any extra whitespace around modification entries is removed, making the parsing more robust to formatting inconsistencies.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.97%. Comparing base (22e6442) to head (79fe780).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #146   +/-   ##
=======================================
  Coverage   71.97%   71.97%           
=======================================
  Files          32       32           
  Lines        3247     3247           
=======================================
  Hits         2337     2337           
  Misses        910      910           

Flag	Coverage Δ
unittests	71.97% <100.00%> (ø)

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