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.
I sent an email about it. The problem is that the flattening would have to be stored using some FP type, this would complicate the Spheroid Concept. But of course you're right. For convenience I added `bg::detail::flattening()` algorithm. Are you ok with this solution?