Add some way to configure which ContextCache multistate samplers use

[jchodera]

It appears that there is no way to configure the ContextCache that the multistate samplers use. Instead, each MCMCMove can optionally be constructed to use its own ContextCache, but the MultiStateSampler will always try to use the global context cache when computing energies.

Instead, it would be useful if we could specify which context cache the MultiStateSampler should use.