WIP: h3 layer

[kylebarron]

cc @felixpalmer

Works in principle with latest deck.gl-layers release.

Change list

• Adds H3HexagonLayer as a core layer type.
• Implements h3 index validation in pure numpy, so that users can have data validated before it goes to JS (where it's hard to surface any data errors)

Todo:

• Implement str_to_h3 vectorized function that converts str input into a uint64 h3 array.
• Implement H3Accessor traitlet that takes in either an array of str or int, validates them, and then packs array as uint64 type to send to the frontend.

Closes #302, for #885