v2: parameter_file -> parameter_files

[dweindl]

Adapt to PEtab-dev/PEtab#641.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.83%. Comparing base (fb36efa) to head (b71db60).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
petab/v2/petab1to2.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #406      +/-   ##
==========================================
- Coverage   73.86%   73.83%   -0.04%     
==========================================
  Files          62       62              
  Lines        6835     6838       +3     
  Branches     1199     1200       +1     
==========================================
  Hits         5049     5049              
- Misses       1328     1329       +1     
- Partials      458      460       +2     

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[dilpath]

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