`multistateanalyzer` fails with 1 unsampled endstate

[dominicrufa]

I think it might be useful to fix the indexing of sampled/unsampled endstates. at present, the multistateanalyzer will fail if there is only 1 unsampled endstate since it assumes there are 2: https://github.com/choderalab/openmmtools/blob/2e64980d86cf087f69e282927eebf2b41b73a3ea/openmmtools/multistate/multistateanalyzer.py#L1510.