Remove GModelRadialRegistry.

The radial registry class has been removed as radial models are now registered using the GModelSpatialRegistry class. There was in fact no need to have a specific radial registry class.
Remove read(), write() and xml_radial() methods.

The reading and writing of model from and to XML files is now handles by the GModelSky base class. This is possible by using the spatial registry class for storing the radial components.
Use GModelSpatialRegistry instead of GModelRadialRegistry.
Remove GModelRadialRegistry.

Radial models are now registered using the GModelSpatialRegistry class. The radial registry class is obsolete.
Add radial models for testing.
Include XML read/write test of radial models.