DESCRIPTION:

Fill the connected area starting at x,y,z. If no sourceset is
given, or it is empty, use the class of the voxel at x/y/z.
All voxels belonging to sourceset that are connected to x,y,z
are remarked to targetset.
The difference to FILLCONNECTED is, that it returns the coordinates
of the found voxel which is closest to dx,dy,dz;