NEW FEATURE: Add > and >> buttons #7

Summary

Add buttons to update dimensions and animate them

Reason

Particularly for time, this better helps to investigate changing behavior of a variable (see also https://github.com/Chilipp/psyplot/issues/10)

Detailed explanation

It is straight-forward to introduce some kind of > and < buttons. >> and << however are more tricky because of the event loop. If we do something like

for t in range(10):
    sp.update(time=t)

we would not see anything updating because matplotlib does the drawing after the loop. Maybe one can circumvent this by spawning the loop in a separate process, but this needs to be tested.

Examples

ncview has buttons like this

Pinging @MuellerSeb