Add NAGLMBISCharges handler for partially polarised NAGL-MBIS charges

[fjclark]

Also add NAGL-MBIS pixi env

[fjclark]

Meant to open this to one of my personal branches -- sorry.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.43697% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.45%. Comparing base (96745a8) to head (f3c1a0d).

Files with missing lines	Patch %	Lines
smirnoff_plugins/collections/charges.py	86.88%	8 Missing ⚠️
smirnoff_plugins/_interchange_patch.py	96.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #120      +/-   ##
==========================================
+ Coverage   86.59%   87.45%   +0.85%     
==========================================
  Files           9       12       +3     
  Lines         694      813     +119     
==========================================
+ Hits          601      711     +110     
- Misses         93      102       +9     

Flag	Coverage Δ
unittests	87.45% <92.43%> (+0.85%)	⬆️

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