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

:x: Patch coverage is 98.55072% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 84.41%. Comparing base (1c969a6) to head (fbd23bf).

Files with missing lines	Patch %	Lines
...penmmforcefields/generators/template_generators.py	98.55%	1 Missing :warning:
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #423      +/-   ##
==========================================
+ Coverage   84.17%   84.41%   +0.23%     
==========================================
  Files           5        5              
  Lines         771      789      +18     
==========================================
+ Hits          649      666      +17     
- Misses        122      123       +1     

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