Should there be an extra context cache clearing in multistatesampler.__del__?

[IAlibay]

Context: From today's OpenFE call, @mikemhenry recommended opening this just to review later.

Quite simply, should there be explit calls to clearing the context cache in __del__ for multistatesampler?

[jchodera]

Yes! We just hadn't run into this before because we don't typically run multiple MultiStateSampler calls in a row, but this may have also been the cause of other issues we had seen earlier.

cc #688