Make all template generators require explicit force field specification

[epretti]

SMIRNOFFTemplateGenerator no longer accepts a default force field specification; this updates the other template generators similarly, as we had previously discussed doing, and fixes an issue suggested by @IAlibay in #434.

The change in the docstring of SystemGenerator is just meant to reflect the change that is already present in #435 but not merged in yet.

There is still a sort of default set within the EspalomaTemplateGenerator: specifically, it uses openff_unconstrained-2.2.1 as the underlying/reference force field. I am not sure if this is appropriate/desired/required behavior or not; a comment from someone more familiar with this would be appreciated.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.48%. Comparing base (5bf74f7) to head (92ca015).

Files with missing lines	Patch %	Lines
...penmmforcefields/generators/template_generators.py	50.00%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #438      +/-   ##
==========================================
- Coverage   84.73%   84.48%   -0.26%     
==========================================
  Files           5        5              
  Lines         799      799              
==========================================
- Hits          677      675       -2     
- Misses        122      124       +2     

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