Fix/strong type checking for cs template parameter

Implement a strict type checking mechanism for the template parameter of the non-Cartesian coordinate systems. The goal is to allow the instantiation of `bg::cs::spherical<T>`, `bg::cs::geographic<T>`, etc., only when `T` is either `bg::degree` or `bg::radian`.

Note that, even with the stronger type checking mechanism mentioned above, since the coordinate system is never really created when a `bg::model::point<>` is created, it is possible to write something like:
```
bg::model::point<double, 2, bg::cs::geographic<double> > p;
```
By making `bg::model::point<>` derive privately from the coordinate system, the above syntax is no longer accepted (see second commit).
Actually the condition in the `MPL_ASSERT` isn't needed since this version will be instantiated for types different than `degree` or `radian`. A simple `false` could be explicitly used there, or does this condition have some other purpose?