Rename GModelSpatialConst.hpp to GModelSpatialDiffuseConst.hpp.
Rename GModelSpatialCube.hpp to GModelSpatialDiffuseCube.hpp.
Rename GModelSpatialMap.hpp to GModelSpatialDiffuseMap.hpp.
Rename GModelSpatialConst.cpp to GModelSpatialDiffuseConst.cpp.
Rename GModelSpatialCube.cpp to GModelSpatialDiffuseCube.cpp.
Rename GModelSpatialMap.cpp to GModelSpatialDiffuseMap.cpp.
Rename GModelSpatialConst.i to GModelSpatialDiffuseConst.i.
Rename GModelSpatialCube.i to GModelSpatialDiffuseCube.i.
Rename GModelSpatialMap.i to GModelSpatialDiffuseMap.i.
Add diffuse spatial model base class.

This is the base class from which all diffuse spatial models will derived.
Correct eval() and eval_gradient() argument.

eval() and eval_gradient() take GSkyDir as argument (was double before due to copy/paste error).
Adapt automatic typecast to new diffuse spatial model definition.
Rename diffuse spatial models to "...SpatialDiffuse..." and derive from GModelSpatialDiffuse.

All diffuse spatial models are now derived from GModelSpatialDiffuse and named GModelSpatialDiffuseXXX.
Add information about spatial model reorganization.
Increment version number.
Adapt tests to reorganized spatial models.
Put back the eval() and eval_grandients() methods in the extension.

The extension is needed to force swig to build the method variants using the GSkydir argument. These methods are implemented in the GModelSpatialRadial class, and are not automatically built by swig.