[Dune] Returning Geometries As Objects

[Carsten Gräser]

Dear 3rd party grid implementors,

this is a warning about a small pitfall with the
new geometry behavior in dune-grid:

Even if you want to use (for a transition) the possibility
to still store references to geometries according to the
temporary default behavior of the new Geometry interface
class you have to adapt your grid.

The reason is that until now you had to store a Geometry<GeometryImpl>
(or maybe a Makeable...<...>) yourself in the Entity. Now you have
to store only the plain GeometryImpl. Using your old implementation
you may end up storing a Geometry<GeometryImpl> which itself stores
a reference to a no longer existing temporary GeometryImpl

You may want to look at the merge commit r7869 as reference for the
neccessary changes.

The reason for warning about this seemingly obvious fact is,
that I just realized that an unmodified grid implementation
might surprisingly still compile and run with the modified grid
interface.

Best,
Carsten