some more regex string fixes; and: use numpy.nan instead of numpy.NAN

[hsk17]

numpy-2 has dropped NAN and NaN
(numpy documentation still says "NaN and NAN are aliases of nan" but this is obviously wrong)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (a3d73b9) to head (51f9add).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
gromacs/fileformats/mdp.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #288   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files         22      22           
  Lines       3954    3954           
=====================================
  Misses      3954    3954           

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[orbeckst]

Thank you for the fix. Sorry, I hadn't seen the PR.

I'll also add you to the CHANGES.

[orbeckst]

I will merge the PR in a few days or sooner if I hear from you. Thank you for your contribution.