GtAssembleVol3D : added code to detect twins for the hexagonal case.

                  Added parameter 'autoload' to defaults to be used in the GtGuiAssembleVol3D.
                  Added parameters 'convention' and 'merge_angle' to defaults to calculate the sigma equivalent values and performs the twin test.
                  Removed parameter 'symm' from defaults.

GtGuiAssembleVol3D : set 'merge_angle' assemble parameter not editable from GUI.