From an email report from a user:

I tried to follow this example:

http://openpathsampling.org/latest/examples/mistis.html#toy-model-mistis

to set up openpathsampling.DirectSimulation for my flux_pairs, but got this error:

AttributeError: 'GromacsEngine' has no attribute 'n_frames_since_start', nor does its options dictionary

Two problems, both solved in this PR:

1. ExternalEngine.n_frames_since_start is only set in its .start() method. It should be set (to None) in __init__.
2. DirectSimulation is not using the .start() and .stop() methods. This didn't matter for OpenMM/toy engine, but does for external engines like Gromacs.

NOTE: I would not recommend using DirectSimulation on external engines. DirectSimulation is mainly useful to get fluxes from toy engine runs or other simulations where you don't want to save trajectory frames. It's better to run a trajectory normally and use TrajectoryTransitionAnalysis.