RBFE protocol: add cpu fallback for minimizing

[IAlibay]

I'm not convinced this is the best approach (actually I think it's kinda broken).

I think ideally we should attempt to do the GPU minimizing, then if the energy is still positive rewind to before we attempted to minimize and default to the CPU. Is this possible?

Ideally we should migrate this out to openmmtools, just adding this here for an initial test/discussion.

Pinging @mikemhenry and @dwhswenson

[codecov]

Codecov Report

Base: 92.44% // Head: 91.63% // Decreases project coverage by -0.81% ⚠️

Coverage data is based on head (26c7bc5) compared to base (f1140ea).
Patch coverage: 2.63% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
- Coverage   92.44%   91.63%   -0.82%     
==========================================
  Files          57       57              
  Lines        4170     4208      +38     
==========================================
+ Hits         3855     3856       +1     
- Misses        315      352      +37     

Impacted Files	Coverage Δ
...fe/protocols/openmm_rbfe/_rbfe_utils/multistate.py	40.45% <2.63%> (-15.46%)	⬇️

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