[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.
Just a couple of additional comments:

1. The black-hatched area covers more than the intersection of the two cells.  In particular, the triangle above the upper green line should not be coloured

2. In an even more complex example there need not be any vertices on the pillars at all.