Added some convenience methods to KeyedChain for symmetry; test simplifications

[dotsdl]

Follow up to #286.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 98.73%. Comparing base (020fac5) to head (efee22e).
Report is 85 commits behind head on main.

Files with missing lines	Patch %	Lines
gufe/tokenization.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #362      +/-   ##
==========================================
- Coverage   98.82%   98.73%   -0.10%     
==========================================
  Files          36       36              
  Lines        2044     2049       +5     
==========================================
+ Hits         2020     2023       +3     
- Misses         24       26       +2     

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