adding charge difference to ligandmapping

[atravitz]

partially addresses #560, along with OpenFreeEnergy/openfe#1479

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Checklist

• Added a news entry

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• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.88%. Comparing base (c053055) to head (e5980ef).
⚠️ Report is 34 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #602   +/-   ##
=======================================
  Coverage   98.87%   98.88%           
=======================================
  Files          39       39           
  Lines        2321     2326    +5     
=======================================
+ Hits         2295     2300    +5     
  Misses         26       26           

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