[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.
> Thanks. I assume that there are 3 faces for both the blue and the green cell on the side that lies on the whiteboard.

In my example, yes. For a simple example with more, divide the blue cell into 40 thin cells.