[[Imported from SVN: r9767]]

For a general grid, such loops are only possible for codimensions 0
and the grid dimension.

[[Imported from SVN: r9766]]

Iterators over entities are only mandatory for vertices and elements.
But here the user may also request indices for other entities.
Hence one cannot use a straight iterator.

[[Imported from SVN: r9759]]

[[Imported from SVN: r9744]]

The globalindex.hh and uniqueentitypartition.hh is from Benedikt Oswald.
The rest is from Benjamin Bykowski, with a few fixes from me.

The globalindex stuff should go into dune-grid eventually, but it needs
some polishing first.

[[Imported from SVN: r9718]]