Simplify Trajectory.__getattr__ - #879

This simplifies Trajectory.__getattr__ and may fix #878.

It adds a small restriction on snapshot "features" -- if you have a snapshot feature named foo, and if you create a function-type snapshot feature called trajectory_foo, then trajectory_foo must be a function that takes a trajectory and returns the values for foo. (I don't think this is going to cause any problems for anyone.)

I'm using those trajectory function features for the following: With the OpenMM engine, a trajectory returns a simtk.unit.Quantity wrapping a numpy array of the results of some snapshot feature, as opposed to making a list/numpy array containing multiple Quantity objects. I've now moved the code that supports this behavior into the OpenMM engine (as trajectory function features specific to it), instead of the base Trajectory object.

This allows us to remove more OpenMM-specific code that made Trajectory.__getattr__ a much more complicated and hard-to-maintain function before.