Feature/geographic

The purpose of this PR is the "official" release of strategies used in geographic CS currently in extensions.

The first commit adds functions and metafunction for accessing radius values and type of various geometries: sphere, spheroid, circle, etc. This is a tweaked version of a part of Nsphere concept (in extensions). It was extended with pointers support and the use of util::bare_type<>. It also adds implementations of 2 models cs::model::sphere<> and cs::model::spheroid<>.

The second commit uses replaces the use of detail::ellipsoid<> with the Spheroid concept.
In the very first versions of Boost.Geometry we used one-character-template arguments (G, D). This is probably still a relict from those days. Later we used full words (Geometry, Dimension).
It's no problem to merge it yet, but for your information.