Sequential ensemble

`SequentialEnsemble` and tests. See #37 for a discussion of the ideas behind this, and the reason to do it.

This PR also includes many corrections to old stuff in ensemble.py (mostly things where skeleton copies hadn't been updated for a new class, or where function hadn't been implemented). It also includes tests for the volume ensembles subclasses. Tests for sequential ensembles include examples of the TIS ensemble and the minus ensemble, testing for correct identification of accepted trajectory against a very large test suite.

This does *not* include cleanup of old code that has been superseded by the `SequentialEnsemble`. I'll leave that around for the time being, so that these additions can get checked in first. (Then we'll prune out the old stuff.)