[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.
Another complementary way to put this is that we return 2 * (interiour faces) + boundary faces. Although I am unsure what is true in parallel?