[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.
Then I still do not get the cornerpoint interface.

Say that I have this setup;
```
_____________
|   0  |  1 |
|      |____|
|      |  2 |
|______|____|
```
How many faces are there for each of the cells? How many vertices contain the interesting vertical ones in the middle?
Why the upper limit of 6?