Some minor bug fixes and towards pymbar-datasets loading

1.  When the first state had zero samples, the free energies would set the reference state to the first state *with* samples.  This was inconsistent with the old behavior, so I have fixed this.
2.  The number of samples in `test_subsampling()` was too small, leading to occasional stochastic failures.
3.  Started work formalizing the munging of datasets from `pymbar-datasets`, which will be coupled with an upcoming PR on that repo.