[Dune] Virtual refinement

[Oliver Sander]

Dear Dune, in particular, Dear Jö!
I just committed a small test for the virtual refinement stuff.
For the pure simplex case, the test simply creates refinements
and then hands over the refined geometry to the regular geometry
test.  That test fails with the message

Error: Methods corner and global are inconsistent.
[repeat ad nauseam]

The method 'corner' is a method I had to add myself to make the
code compile at all.  However, all I did was to copy the method
operator[], which I expected to do the right thing.  Apparently
I was wrong, there is a bug somewhere, but my limited knowledge
of the refinement code stops me from finding it.

@Jö, could you please have a look?

Thanks,
Oliver