Create nonequilibrium multi state sampler

[zhang-ivy]

We want to create a NonequilibriumMultiStateSampler that takes in 2 states (and optionally 2 auxiliary states with LRC fully on) and run nonequilibrium switching.

The class should optionally accept rest caches (instead of running equilibrium MD). It should also optionally only run one direction of switching (instead of a full cycle of forward and reverse).

Example scripts:

• Running vanilla neq: https://github.com/zhang-ivy/perses_protein_mutations/blob/master/code/31_rest_over_protocol/run_neq_vanilla.py
• Starting with a rest cache: https://github.com/zhang-ivy/perses_protein_mutations/blob/master/code/31_rest_over_protocol/run_neq_rest_over_protocol_save_solvent_last_frame.py
• Starting with a rest cache and running a single direction: https://github.com/zhang-ivy/perses_protein_mutations/blob/master/code/31_rest_over_protocol/run_neq_rest_over_protocol_save_solvent_last_frame_bridge_direction.py

[ijpulidos]

@jchodera Tagging you again on this one, when you have the chance. Thanks!