Friendly ID property for clips

[tswast]

I'm finding it useful to create helper variables to keep track of which clip is which.

pikov/examples/gamekitty/gamekitty_transitions.ipynb

Line 41 in 279cf9b

"sitting = gamekitty.get_clip(1)\n",

Such a feature could be built-in to the clip table, where perhaps clips could be accessed by name in addition to by numeric ID. It could work something like:

gamekitty = pikov.Pikov.open('gamekitty.pikov')
clips = gamekitty.clips
stand_up = clips['stand_up']