Implementation of WaterSelection for water resiude selection

[talagayev]

Fixes #4839 through the addition of the water token.

Changes made in this Pull Request:

• Addition of WaterSelection class with the water token to select water residues
• Addition of TestSelectionsWater with tests to cover water token in test_atomselections.py

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

Developers certificate of origin

• I certify that this contribution is covered by the LGPLv2.1+ license as defined in our LICENSE and adheres to the Developer Certificate of Origin.

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📚 Documentation preview 📚: https://mdanalysis--4854.org.readthedocs.build/en/4854/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.63%. Comparing base (9110a6e) to head (92aa02f).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4854      +/-   ##
===========================================
- Coverage    93.65%   93.63%   -0.03%     
===========================================
  Files          177      189      +12     
  Lines        21779    22854    +1075     
  Branches      3064     3064              
===========================================
+ Hits         20398    21400    +1002     
- Misses         929     1002      +73     
  Partials       452      452              

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