Speed up `utils.map_drawn_samples`

[rmcgibbo]

• This function is slow. See https://mailman.stanford.edu/pipermail/msmbuilder-user/2015-April/000070.html
• For many trajectory formats like xtc, seek is not an O(1) operation, so repeatedly calling load_frame() to load a single frame (random access) is slow. If you need to load multiple frames from the file, it would be much better to sort the indices, load the trajectory, and then run through it and grab all of the frames.
• Also, the name "map_drawn_samples" is really awkward. Maybe we can come up with a better one?