Add optional emle_plus support to OpenFFCreationStrategy

[JMorado]

This PR adds an emle_plus parameter to the OpenFFCreationStrategy.create() method, enabling users to optionally use experimental versions of emle-engine (viz. EMLE+) instead of the upstream emle-engine code.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
/home/runner/micromamba/envs/gha-test-env/lib/python3.12/site-packages/fes_ml
fes.py	167	113	32%	104, 110–111, 115–131, 135–138, 142–146, 198, 233–247, 278–290, 322–331, 359–376, 404–413, 444–497, 517–532
log.py	30	2	93%	75–76
mts.py	55	45	18%	19–20, 60–84, 109–147
utils.py	32	18	44%	40, 56–57, 72–92
/home/runner/micromamba/envs/gha-test-env/lib/python3.12/site-packages/fes_ml/alchemical
alchemical_factory.py	20	1	95%	75
alchemical_state.py	19	6	68%	51–56
alchemist.py	139	38	73%	44, 49, 62, 66–79, 83, 106, 128, 156, 184, 192–193, 200, 203–222, 229–230, 250–256, 262
/home/runner/micromamba/envs/gha-test-env/lib/python3.12/site-packages/fes_ml/alchemical/modifications
base_modification.py	56	21	62%	73, 89, 103–106, 118–121, 135–138, 150–153, 176, 200–203, 217, 243
charge_scaling.py	26	3	88%	66, 68–69
charge_transfer.py	57	40	30%	26, 60–84, 125–186
custom_lj.py	40	24	40%	32, 87–151
emle_potential.py	62	47	24%	35, 108–209
intramolecular.py	120	9	92%	144–150, 176, 315
lj_softcore.py	56	4	93%	96, 121, 152, 160
ml_correction.py	29	13	55%	35, 80–97
/home/runner/micromamba/envs/gha-test-env/lib/python3.12/site-packages/fes_ml/alchemical/strategies
base_strategy.py	58	8	86%	29, 71, 122, 185–190
openff_strategy.py	276	226	18%	100–103, 129–162, 188–206, 225–234, 253–264, 268, 290–327, 346–356, 381–393, 410–423, 521–778
sire_strategy.py	70	9	87%	139–143, 176, 186–187, 204
TOTAL	1417	627	56%

Tests	Skipped	Failures	Errors	Time
3	0 💤	3 ❌	0 🔥	3m 19s ⏱️

[JMorado]

Related to #18.