Move settings from SystemSettings to ForcefieldSettings

[hannahbaumann]

As discussed in a power hour, moving the nonbonded_method and nonbonded_cutoff to ForcefieldSettings.

[pep8speaks]

Hello @hannahbaumann! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file gufe/tests/test_transformation.py:

Line 34:80: E501 line too long (396 > 79 characters)
Line 131:80: E501 line too long (435 > 79 characters)

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (1f77481) 99.22% compared to head (3eec306) 98.97%.

Files	Patch %	Lines
gufe/settings/models.py	72.22%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #276      +/-   ##
==========================================
- Coverage   99.22%   98.97%   -0.25%     
==========================================
  Files          36       36              
  Lines        1938     1956      +18     
==========================================
+ Hits         1923     1936      +13     
- Misses         15       20       +5     

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