Add add_library_charges_to_forcefield fn

[fjclark]

Description

Add add_library_charges_to_forcefield convenience function for writing partial charges from a Molecule to library charges. Recommended over https://github.com/openforcefield/openff-toolkit/blob/120f71473a4b87cb314bd7acc706ce7e9ffdeda4/openff/toolkit/typing/engines/smirnoff/parameters.py#L3009 because it uses the MergeQueryHs trick to ensure fast matching.

Addresses #64.

Status

• Ready to go

[fjclark]

@jthorton, if you're still interested in this, does this seem like a reasonable solution? I'm keen to keep everything in the force field as I think this is cleaner, rather than using the partial charges in a Molecule directly. This means a fit can be reproduced from the force field file and presto settings file. Avoiding using the partial charges directly seems in keeping with the argument in openforcefield/openff-toolkit#806 (comment). Thanks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.85%. Comparing base (28483fb) to head (a20b527).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
+ Coverage   92.77%   92.85%   +0.08%     
==========================================
  Files          26       26              
  Lines        2810     2843      +33     
==========================================
+ Hits         2607     2640      +33     
  Misses        203      203              

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