Turn AFE protocols into multiple units

[IAlibay]

Blocked by #1775
Fixes #1724

TODO:

• Run and fix the slow & integration tests
• Get new serialized results for ahfe and abfe protocols

Checklist

• All new code is appropriately documented (user-facing code must have complete docstrings).
• Added a news entry, or the changes are not user-facing.
• Ran pre-commit by making a comment with pre-commit.ci autofix before requesting review.

Manual Tests: these are slow so don't need to be run every commit, only before merging and when relevant changes are made (generally at reviewer-discretion).

• GPU integration tests
• example notebook testing
• packaging tests: run this for any large feature PRs or PRs that add test data.

Developers certificate of origin

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

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 97 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.06%. Comparing base (990b77b) to head (281adb5).

Files with missing lines	Patch %	Lines
openfe/protocols/openmm_afe/base_afe_units.py	86.74%	22 Missing ⚠️
...enfe/tests/protocols/openmm_abfe/test_abfe_slow.py	0.00%	19 Missing ⚠️
...enfe/tests/protocols/openmm_ahfe/test_ahfe_slow.py	0.00%	19 Missing ⚠️
openfe/protocols/openmm_utils/serialization.py	51.42%	17 Missing ⚠️
...penfe/protocols/openmm_afe/afe_protocol_results.py	90.54%	14 Missing ⚠️
openfe/protocols/openmm_afe/abfe_units.py	97.60%	3 Missing ⚠️
.../tests/protocols/openmm_abfe/test_abfe_energies.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1776      +/-   ##
==========================================
- Coverage   95.40%   93.06%   -2.35%     
==========================================
  Files         190      197       +7     
  Lines       16818    17106     +288     
==========================================
- Hits        16046    15920     -126     
- Misses        772     1186     +414     

Flag	Coverage Δ
fast-tests	93.06% <90.90%> (?)
slow-tests	?

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[IAlibay]

pre-commit.ci autofix

[IAlibay]

pre-commit.ci autofix

[github-actions]

No API break detected ✅