Allow residue X with MassModification to indicate gap of known mass

[RalfG]

Fixed

• Peptidoform: Allow residue X with MassModification to indicate gap of known mass (see ProForma 4.2.7). Previous behavior would raise AmbiguousResidueException for amino acid X. Now it is allowed when combined with a mass modification (fixes Cannot compute theoretical mass for non-natural residue Aib using [MOD:01001] #127).

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.23%. Comparing base (4742ee0) to head (4b49b00).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
psm_utils/peptidoform.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   70.88%   72.23%   +1.34%     
==========================================
  Files          32       32              
  Lines        3208     3216       +8     
==========================================
+ Hits         2274     2323      +49     
+ Misses        934      893      -41     

Flag	Coverage Δ
unittests	72.23% <80.00%> (+1.34%)	⬆️

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