Add WMTSArtist and GeoAxes.add_wmts().

Open issues (~~some perhaps~~ out of scope):
- ~~Thin seams sometimes visible between tiles.~~
- Does not respect limits of layers which only provide a subset of tiles (via. TileMatrixLimits). (This is a limitation of owslib.)
- Coping with CRSs with YX axis order, such as EPSG 4326 (lat-lon).

See #405.