Add trajectory sorting function

[chryswoods]

A suggestion prompted by a reviewer - add a function that sorts loaded trajectory frames in different ways, e.g. could sort based on the timestamp of the trajectory files, the name of the files, or even looking at the simulation times in the loaded frames and reordering so that they are in simulation time order. This way, a user wouldn't have to think about arranging the files.

Especially for simulation times, this would be relatively straightforward to do - just check the simulation time from each trajectory file and then load them in simulation order.