feat: add abfe gathering support first draft

[atravitz]

First pass at adding an MVP for supporting abfe results gathering, based on https://docs.openfree.energy/en/latest/tutorials/abfe_analysis_tutorial.html

Note that this is targeting feat/cli-gather-improvements, not main, and so will get a lot of code polish after this initial PR.

Checklist

• [ ] Added a news entry (I'll add a news entry once this is more polished)

Developers certificate of origin

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

[codecov[bot]]

Codecov Report

:x: Patch coverage is 87.20000% with 16 lines in your changes missing coverage. Please review. :warning: Please upload report for BASE (feat/cli-gather-improvements@dfee883). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
openfecli/commands/gather_abfe.py	85.04%	16 Missing :warning:

Additional details and impacted files

@@                       Coverage Diff                       @@
##             feat/cli-gather-improvements    #1686   +/-   ##
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  Coverage                                ?   92.94%           
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  Files                                   ?      187           
  Lines                                   ?    16313           
  Branches                                ?        0           
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  Hits                                    ?    15162           
  Misses                                  ?     1151           
  Partials                                ?        0           

Flag	Coverage Δ
fast-tests	92.94% <87.20%> (?)

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