GammaLib

So far, @GResponse@ only handles point sources, radial models and diffuse models. The following mapping exists:
* Spatial models of type @GModelSpatialPtsrc@ call @GResponse::irf_ptsrc@ and @GResponse::npred_ptsrc@
* Spatial models of type @GModelRadial@ call @GResponse::irf_extended@ and @GResponse::npred_extended@
* All other models call @GResponse::irf_diffuse@ and @GResponse::npred_diffuse@

To accomodate for elliptical models, a more direct mapping should be implemented:
* @GModelSpatialPtsrc@: call @GResponse::irf_ptsrc@ and @GResponse::npred_ptsrc@
* @GModelRadial@: call @GResponse::irf_radial@ and @GResponse::npred_radial@
* @GModelElliptical@: call @GResponse::irf_elliptical@ and @GResponse::npred_elliptical@
* All other models call @GResponse::irf_diffuse@ and @GResponse::npred_diffuse@

Since this implies an interface change, all derived response classes need also to be adapted.