Add a cubed sphere (#261)

* extend generate_icosphere to tensor product groups

* renamed to generate_cube_surface and add to docs

* rename to generate_sphere and deprecate

* remove usages of deprecated generate_icosphere

* add docs for group_cls in generate_sphere

* actually add deprecation warning

* Drop generate_icosphere from docs

Co-authored-by: Andreas Klöckner <inform@tiker.net>