Add initial v-site support

[SimonBoothroyd]

Description

This PR adds support for virtual sites, and re-works how parameters are applied to support change increments.

Status

• Ready to go

[codecov]

Codecov Report

Merging #57 (ef2a25d) into main (9d56166) will increase coverage by 1.85%.
The diff coverage is 99.12%.

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
+ Coverage   96.70%   98.55%   +1.85%     
==========================================
  Files           9       11       +2     
  Lines         334      485     +151     
==========================================
+ Hits          323      478     +155     
+ Misses         11        7       -4     

Flag	Coverage Δ
unittests	98.55% <99.12%> (+1.85%)	⬆️

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Files Changed	Coverage Δ
smirnoffee/ff/__init__.py	100.00% <ø> (ø)
smirnoffee/potentials/_potentials.py	94.44% <90.00%> (+0.32%)	⬆️
smirnoffee/ff/_ff.py	98.84% <98.63%> (+8.84%)	⬆️
smirnoffee/ff/nonbonded.py	100.00% <100.00%> (ø)
smirnoffee/ff/valence.py	100.00% <100.00%> (ø)
smirnoffee/potentials/__init__.py	100.00% <100.00%> (ø)
smirnoffee/potentials/nonbonded.py	100.00% <100.00%> (+4.16%)	⬆️
smirnoffee/potentials/valence.py	88.88% <100.00%> (ø)
smirnoffee/utils.py	100.00% <100.00%> (ø)

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