Percolator fixes: ScanNr writing and inferring style from file extension

[RalfG]

Fixed

• io.percolator: Fix and improve ScanNr inferring and writing
• io.percolator: Infer style from file extension if not provided (enables dynamic style determination in, for instance, convert function).

[codecov]

Codecov Report

Attention: Patch coverage is 66.10169% with 20 lines in your changes missing coverage. Please review.

Project coverage is 65.96%. Comparing base (50f9a9c) to head (711fd10).
Report is 4 commits behind head on main.

Files	Patch %	Lines
psm_utils/io/percolator.py	65.51%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #94      +/-   ##
==========================================
+ Coverage   65.19%   65.96%   +0.77%     
==========================================
  Files          26       26              
  Lines        2508     2533      +25     
==========================================
+ Hits         1635     1671      +36     
+ Misses        873      862      -11     

Flag	Coverage Δ
unittests	65.96% <66.10%> (+0.77%)	⬆️

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