Added animation hold

[colleenjg]

Added an option when creating animations (trajectory or rate timeseries) to hold the last frame for a few seconds.

You'll notice that I made frames an array instead of a lazy iterable.
frames = np.concatenate([np.arange(frames), np.repeat(frames - 1, hold_n)])

This is not the most memory efficient approach, but I couldn't find a way to make frames a lazy iterable. itertools.chain can chain iterables, but then you can't call len() on them, which matplotlib's animation function needs to do. This seemed like a reasonable compromise. If the user sets hold to 0, frames is left as a lazy iterable so previous behaviour is unchanged.

[TomGeorge1234]

honestly it seems fine to me! We can always look into it again if the inefficency becomes an issue.

thanks!