In particular this means having to carry mappers for the local indices around.
This makes the code more verbose, but it also makes it easier to see what is
happening.  While coding this stuff I keep having doubts about whether all
the renumbering works correctly...

[[Imported from SVN: r9925]]

[[Imported from SVN: r9923]]

The local copy was to get the thing developed and working.  Now that it is
working we upstream it into dune-grid.

[[Imported from SVN: r9922]]

The IndexSet interface class from dune-grid doesn't have it, and we want
to be like the interface method as much as possible.

[[Imported from SVN: r9921]]

[[Imported from SVN: r9920]]

That's what this method is called in the dune-grid IndexSet
interface.

[[Imported from SVN: r9919]]

[[Imported from SVN: r9918]]

[[Imported from SVN: r9917]]

Yes it is to be moved to dune-grid, but for the transition it is
easier to use the one from this module.

[[Imported from SVN: r9916]]

[[Imported from SVN: r9915]]

[[Imported from SVN: r9902]]

[[Imported from SVN: r9901]]

[[Imported from SVN: r9882]]

We are using stuff from there.

[[Imported from SVN: r9880]]

[[Imported from SVN: r9844]]

[[Imported from SVN: r9840]]

[[Imported from SVN: r9767]]

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

[[Imported from SVN: r9766]]

[[Imported from SVN: r9764]]

[[Imported from SVN: r9763]]

GlobalP2Mapper is intended to be for P2 finite element spaces, what
GlobalIndex is for P1 spaces: a numbering for all degrees of freedom
that is globally unique across all processors.

[[Imported from SVN: r9761]]

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]]

No idea why valgrind didn't complain about this.

[[Imported from SVN: r9758]]

[[Imported from SVN: r9749]]

And use it in one place in VectorCommunicator

[[Imported from SVN: r9748]]

[[Imported from SVN: r9747]]

[[Imported from SVN: r9746]]

[[Imported from SVN: r9744]]

Much easier to use than the previous API.

[[Imported from SVN: r9735]]

[[Imported from SVN: r9733]]

[[Imported from SVN: r9732]]

Those do many-to-one communication with a single method call.
Previously, two subsequent calls were needed.  There is no reason
why the interface has to consist of two calls.  A single one is
much easier to use.

[[Imported from SVN: r9730]]

That way we can handle multigrid transfer matrices as well.

[[Imported from SVN: r9719]]

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]]