Rename irf_extended to irf_radial and npred_extended to npred_radial.
Adapt to new spatial model organisation.

Renamed irf_extended to ird_radial and npred_extended to npred_radial. Renamed also the radial kernel integration methods to npred_radial_kern_theta and npred_radial_kern_phi. Handle properly different spatial model components: point source, radial source, elliptical source, diffuse source.
Implement npred_elliptical method.