The GSource value constructor takes now a GModelSpatial pointer instead of a const reference, and the model is no longer cloned.

Model cloning is time consuming as it implies dynamic memory allocation, and the GSource class is used for very low-level functions in the model computation, so we don't want to have a speed penalty here. Therefore, GSource now carries the model information instead of cloning it. If cloned information is needed, one can alway pass a cloned pointer to GSource.
Adapt to new GSource interface.
Do not allocate or deallocate a spatial model as the GSource class just carries the external pointer.
Remove index check from operator[] and add at() methods that allow vector element access with index check.

We follow now the standard of the std::vector library that provides no index check for operator[] access and index check for at() access methods.