Refactor GeoAxes gridliner handling and format flow; remove cartopy monkey patches (#454)

* Refactor geo gridliner adapters

* Add adapter-focused geographic tests

* Refine geo gridliner helpers and constants

* Add gridliner adapter tests

* Refactor cartopy gridliner overrides

* Tweak gridliner subclass wording

* Refactor GeoAxes.format into helpers

* Document GeoAxes.format flow

* Restore gridliner toggle call for empty labels

Author: cvanelteren