Implement a GlobalIndexSet class

The class implements an index set over all entities in a distributed grid.
This patch introduces the code as presented in FS 1489, with minor modifications.
It is fully functional (as far as I can tell).

Future patches will improve the interface.

Thanks to Benedikt Oswald and Patrick Leidenberger for donating the original
version of this code.
Add a unit test for the new GlobalIndexSet class
Remove the getGridView method -- real index sets don't have it either
[cleanup] Remove unnecessary template parameters from the IndexExchange class
[cleanup] Merge the codim-0 IndexExchange class into the general one
Merge the Codim-0 specialization into the general class

Which is actually possible without making the code particularly difficult,
and without decreasing efficiency, either.  The new code is much shorter.
Also, getting rid of the specialization is a prerequisite for making
CODIM a run-time parameter.
Remove the 'localIndex' method taking an Entity

That method simply calls the 'index' method on the grid's index set.
The user may call that method directly.
Remove the gridview_ data member from the UniqueEntityPartition class

It is only used in the constructor.  The only use outside of the constructor
is not used itself, and can be removed, too.
spelling
Construct a UniqueEntityPartition only when CODIM != 0

Because for the CODIM==0 case it is not needed.
[cleanup] Remove obsolete comment
[cleanup] Always use Entity::subEntities instead of Entity::count
Make the GlobalIndexSet codimension a run-time parameter

I see no need to have it at compile time.
Use the Index type exported by the grid index set, rather than 'int'
Rename the method 'globalIndex' to 'index'

That is the name used in the official IndexSet interface.
Remove the localIndex method taking an integer index argument

Computing from global indeces to some sort of local ones is not the job
of an dune-grid-style index set class.
Import the type 'Priorities', which is used by UG for what Dune would call 'PartitionTypes'
Include missing header deprecated.hh
Implement the GlobalIndexSet::subIndex method
Remove the method 'index' taking a local index as an argument

The dune-grid IndexSet class doesn't have it.  Besides, it is not clear what
this method should do in case there are different elements.  Then, there
would be more than one element associated to a given index.