Fast ensemble check - #669

This adds a new argument to Ensemble.__call__, called candidate. If candidate is True, then shortcuts can be used to calculate whether the trajectory is in the ensemble. The assumption is that this is a "candidate trajectory," as described in path ensemble theory; i.e., a trajectory that could have been generated by shooting.

For example, this will allow us to assume that all frames except first and last are not in any state for a (flexible length) TPS ensemble or a TIS ensemble. For ensembles where there is no shortcut to be had, candidate just has no effect.

This is the first step toward using the generalized CVs of #662 in analysis (simply need to replace the TISEnsemble's use of max(orderparameter(trajectory)) by a CV that stores that result). That will be a huge improvement on analysis speed. If the max lambda is known, a TIS candidate trajectory can be checked by looking at the first frame, the last frame, and the max lambda. Currently, we loop over all frames, find the max lambda, and check that only the first and last are actually in the state. This will also make a big difference for sampling speed for toy models.

Note that we'll still have the occasional sanity_check during simulation, which will not use the candidate assumption. This is just to make sure that nothing goes terribly awry during the sampling.

This candidate approach is very similar to what we already had as trusted for can_append and can_prepend. Unfortunately, there appear to be a couple cases where the __call__ with trusted=True is being already being used in a way that would conflict with replacing __call__'s trusted with what I'm doing with candidate, so it looks like candidate might be separate from trusted (Annoying, because they're almost the same, and __call__ with trusted=True almost never happens. When it does, it is deep in SequentialEnsemble, so there might be a way to fix it, but it'll take some thought.)

• add candidate flag to Ensemble.__call__
• add speed-up for TISEnsemble using candidate
• tests for TISEnsemble using candidate (including where candidate is too trusting)
• add the use of candidate in replica exchange checks, path reversal checks