SMIRNOFF: support virtual sites, multiple force field files

[epretti]

Adds support for SMIRNOFF virtual sites. Still draft quality; needs more testing for loading multiple OFFXML force field files simultaneously (this was necessary for some virtual site tests)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 97.46835% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.36%. Comparing base (fb8ed0e) to head (59c2527).

Files with missing lines	Patch %	Lines
...penmmforcefields/generators/template_generators.py	97.46%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #423      +/-   ##
==========================================
+ Coverage   84.17%   84.36%   +0.18%     
==========================================
  Files           5        5              
  Lines         771      793      +22     
==========================================
+ Hits          649      669      +20     
- Misses        122      124       +2     

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