GeoViews integration

[jbednar]

@schaunwheeler , it was fun to talk to you today! For some reason I'm not able to send our meeting notes by email (it just bounces!), so I'm filing this issue, but maybe it's information generally useful to Theto users anyway. In any case, Schaun met with me and @philippjfr to discuss possible synergies between Theto and GeoViews, given that both of these tools offer Bokeh-based visualization tools for shapes in a geographic context.

We agreed that theto provides several types of functionality not currently available from GeoViews and its companion libraries, and to record those we filed some issues on our projects:

geoviews#372: Allow data to be provided as geohashes
geoviews#371: Document how to use well-known text with GeoViews
holoviews#4079: Generate Panel widgets for selecting between sub-elements automatically

Since then, Philipp has already made a stab at geohash support (holoviz/geoviews#373), and the WKT support is just a matter of documentation rather than coding, which leaves the last item (widgets) as something for us to address, and it's one that I'm very excited about. Thanks for releasing Theto and inspiring us!