Forward more read and write methods from CPGridData to CpGrid that are needed.
Added routine to compute overlap region.
[cpgrid] Added routine to redistribute grid (not working yet!)
Correctly initialize index set and id set with default constructor.
[entity] export codim within EntityRep
Added a class representing a global id set of DUNE.

The global ids are stored in vectors that can be accessed
by the index of a entity. As a fallback we use the local id set to
compute the id.
Computed and added the global id set to the CpGridData.
Use dune-common's nullptr compatibility
[cpgrid] Added method to scatter a global grid to all processors and corresponding test.
Added rules to compute the partition types and a distribution test.
Implements iterators for different partitiontype with preliminary tests.
Corrected filename in comment.
Throw a runtime error on global methods when distributed.

Both IJK and globalCell do not make sense for a distributed grid.
Therefore we throw a runtime error in this case.
Meaningful implemented methods to get the size of the partitions.