Allow for any barostat in SystemGenerator

[hannahbaumann]

As described in this issue (#413) SystemGenerator currently only supports the MonteCarloBarostat.
Here, instead of re-creating the barostat in _modify_forces, the barostat is copied, however, there may be reasons why it was necessary to re-create the barostat?

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.17%. Comparing base (839f225) to head (4d11936).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #414      +/-   ##
==========================================
+ Coverage   84.14%   84.17%   +0.02%     
==========================================
  Files           5        5              
  Lines         776      771       -5     
==========================================
- Hits          653      649       -4     
+ Misses        123      122       -1     

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[epretti]

Sorry this was sitting here for a bit; I forgot someone needed to press a button to actually make the CI run. This looks good to me provided the tests are passing. Is it ready or still WIP?

[mattwthompson]

I think we should have at least one test: one that takes in a barostat (ideally of a different type than the default) and make sure that it comes out as expected

[epretti]

Good point; I saw we already have a test here

openmmforcefields/openmmforcefields/tests/test_system_generator.py

Line 142 in 839f225

def test_barostat(self):

but it only tests the standard MonteCarloBarostat type. Any type of barostat should now work but it'd probably be good to have the test cover one of the other types.

[hannahbaumann]

Thanks! I added the MonteCarloMembraneBarostat to the test!

[epretti]

Looks alright to me, although I haven't worked a lot on SystemGenerator so I might be missing something: @mattwthompson anything else that stands out to you that should be addressed?

[mattwthompson]

I don't use SystemGenerator but this does basically what I expected and tests the appropriate behavior. I would be surprised if this breaks something, but if it does that would indicate something isn't tested and we can patch that later