Change reference_sphere_tag and reference_spheroid_tag to srs_sphere_tag
and srs_spheroid_tag respectively.
Adapt algorithms, strategies and tests to the new namespace and tags.
Add new tags for reference sphere/spheroid concepts.
Add cs::model::sphere<> and cs::model::spheroid<>.
Add get_radius(), set_radius() and radius_type<> functions/metafunctions.
Add tests.
Refactor [extensions] nsphere to use the official functions.