[doc] Improves the doxygen documentation of CpGrid.

This commit conceptually does two things:
1. It documents the so-called simple grid interface.
2. It groups the methods into the following logical groups:
   * DUNE grid interface
   * IO routines
   * Simple grid interface
   * Cartesian grid extensions
   * Parallel grid extensions

This should make it much easier for future and current contributors
to grasp the code.
I am not sure this is correct. The "length = face area" convention comes from MRST, and is therefore also used in the UnstructuredGrid interface, but for Dune it is explicitly a unit vector that is used. I got confused trying to trace the normal through the CpGridData class, so could you check this?