[Dune] Dune::Geometry::operator=(...)

[Christian Engwer]

Hi,

for Grid and Entity we already decided to prevent the assignment and
unassigned instances by declaring the empty constructor and the
assignment operator private.

The Geometry is quite a similar case and we in Heidelberg think it
would be strange to allow the assignment for Geometries. As we haven't
discussed this case yet I would suggest to handle the Geometry in the
same way as Grid and Entity.

Any objections? Otherwise I will change the test and the headers.

Cheers Christian