With exact treatment of PME electrostatics, computing the energy matrix required for MBAR is very expensive. We can compute the optimization discussed in choderalab/yank#678 and choderalab/sams#9: i.e., split the alchemical forces into different groups and compute the energy of only the groups that are different. Here's how I plan to add the feature:

• Add option split_force_groups in AbsoluteAlchemicalFactory.__init__() to split forces by alchemical variable.
• Require ComposableStates to implement a method _find_force_groups_to_update(context, context_thermodynamic_state) that figures out which force groups need to be computed to obtain the difference of potential energy.
• Add a classmethod ThermodynamicState.reduced_potential_at_states(context, thermodynamic_state_list) that uses the method above to efficiently compute the reduced potential at all the given thermodynamic states.