Add `GeoAxes.add_wmts()`.

Simple, proof-of-concept implementation of WMTS support on a GeoAxes.

Open issues (some perhaps out of scope):
 - The API (i.e. `GeoAxes.add_wmts()`) and how it relates to the existing API (e.g. `GeoAxes.add_feature()`) and future WxS integration.
 - Thin seams sometimes visible between tiles.
 - Does not respect limits of layers which only provide a subset of tiles (via. TileMatrixLimits).
 - Coping with CRSs with YX axis order, such as EPSG 4326 (lat-lon).

Also:
 - Not an interactive implementation - it will not request more tiles as you zoom in on an interactive plot.

See #405.