indexable graphic attributes

[kushalkolar]

implements #77

GraphicFeatures to implement:

• colors
• sizes
• data - mostly wrap current update_data methods, just pass the slice obj directly to geo.grid or geo.positions, and for positions update_range with indices parsed by slice object, otherwise entire thing with grid.
• present (different from visibility) - not indexable
• events for when these change

[kushalkolar]

color indexing is really nice :D

crazy_colors-2022-12-19_04.30.30.mp4

[kushalkolar]

this insanity is possible now: https://www.youtube.com/watch?v=j_gwi-Wf1Ao

also this which is more useful 😄

[kushalkolar]

For scatter we can make size an indexable feature too.

[kushalkolar]

@EricThomson, @clewis7 , @ArjunPutcha
guys this is pretty epic, changing a GraphicFeature can trigger events 😄

Caitlin, pick_info formatting is the same as pygfx events to make life easy :D

features-events-2022-12-22_06.07.58.mp4

Will merge as soon as #61 is ready

[EricThomson]

Nice -- this will make certain other things so much easier.