First implementation of a parallel dune-cornerpoint grid

This is basically a rebased version of my parallel branch. Comparison should be easy except for Entity.hpp where someone exchanged tabs with 8 spaces. A I usually set the tab-width to 4 the formatting is messed up in this header. I did not fix the fromatting to make reviewing the patches not more difficult. Of course I am willing to fix this on request.

The current implementation is still a bit limited in the following sense:
One needs to setup a global grid on each processor, e.g. by loading an eclipse file. Using CpGrid::loadbalance this grid will be distributed to to a parallel representation. Now it can be used for parallel computations. There is currently no way of setting up a parallel distributed grid (not even a cartesian one).

Currently there is no way of gathering the data from the parallel computation to the global representation on each processor. This is needed for OPM various output routines that usually convert it to an unstructured grid representatioin. This means you can compute but you cannot write out data the usual way.

Currently this code is only tested by the grid checks within dune-grid (which unfortunately require additional patches). So there might be unrevealed bugs.

Having said that, it seems suitable to me to merge these changes now as the sequential version of dune-cornerpoint grid should work as before.