Add explicit solvent component

[atravitz]

closes #299

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• Added a news entry

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 98.25%. Comparing base (d077d7a) to head (8634cda).

Files with missing lines	Patch %	Lines
gufe/components/explictsolventcomponent.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #453      +/-   ##
==========================================
- Coverage   98.52%   98.25%   -0.28%     
==========================================
  Files          39       40       +1     
  Lines        2166     2172       +6     
==========================================
  Hits         2134     2134              
- Misses         32       38       +6     

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