[ENH] include Basemap options for choropleth visualisation

[slumnitz]

Add option Basemap=TRUE, tiles=exampletile to function call for choropleth visualisations
Add @darribas's contextily as soft-dependancy to splot.

[sjsrey]

would Basemap=FALSE be preferable as the default?

[slumnitz]

yes Basemap=FALSE would be the default.
Alternatively, we could think about an option to set a Basemap for the whole jupyter kernel? So every map in the nb is automatically displayed with the set Basemap? similar to set_style() in matplotlib?

[darribas]

Wohooo! This would be awesome! The main challenge I think refers to CRSs (what else...). We're currently trying to figure out a way to add tile warping so data don't need to be reprojected. It's a bit hairy... The way we currently work with now is we just support Web Mercator, so the user has to rely on geopandas or equivalent to transform beforehand. Would this method expect a GeoDataFrame? If so, we could work with that under the hood.

This will be real cool!!! 🎉