[Dune] Returning Geometries As Objects

[Christian Engwer]

> If I'm not mistaken, the following people have agreed on (b') by now:
> Carsten, Jö, Martin, Oliver.
> This means, we're still two developers short on an actual decision.

I thought I said that Carstens suggestions (aka b') is fine by me.

> Nevertheless, I will proceed as announced, i.e., change the branch to
> (a) documentat that the returned geometry object has to be valid
> until the next grid modification,
> (b) let the constructor of Dune::Geometry give a deprecation
> warning, if StoreGeometryReference::v is true, thus issueing a
> warning to the user.
> 
> Afterwards I will silently merge the branch into the trunk this
> afternoon (unless, of course, a developer tells me not to).

As I have the impression that we have come to a common ground for the
desired features, I think it is good to merge to avoid further
conflicts. We have then to concentrate on updating the grid
implementations.

Cheers
Christian