implemented load, eval and eval_gradients methods in GModelSpatialDiffuseCube
Update after implementation of feature #944.
write methods now correctly sets the file attribute.
Add test file for diffuse map cube.
Set filename to new diffuse cube test file.
Adapt for existence of diffuse cube test file.
Load the dummy diffuse test cube file. The content of the file is not relevant as it is actually not checked. It just has to be made sure that a file exists.