Added wms to GeoAxes.
Implements WMS as a RasterFetcher and adds the SlipperyImageArtist for
RasterFetcher interactive visualisation.
Clear image accumulator to avoid ghosting.
Merge pull request #416 from rhattersley/wmts-ghosts

Clear image accumulator to avoid ghosting.
Updated documentation of get_extent.
Merge pull request #418 from pelson/get_extent_doc

Updated documentation of get_extent.
Review actions.
Renamed SRC.
Merge pull request #415 from pelson/wms_artist

GeoAxes.add_wms()
Port WMTS rendering to GeoAxes.add_raster().
Update docs and WMS tests.
Add/update WMTS tests.
Fix typo raising NotImplementedError.
Minor tweaks.
PEP 8 fix.
Merge pull request #420 from rhattersley/wmts-refactor

Port WMTS rendering to GeoAxes.add_raster().
Composite onto white instead of red.
Support for WMTS TileMatrixSetLimits.