[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.
Assuming the above is a view from the side, cells 1 and 2 have 4 faces, cell 0 has 5 faces. The two faces in the middle would have 4 vertices each. I have drawn a more complicated example.

![cornerpoint-intersection](https://cloud.githubusercontent.com/assets/1876621/5338635/7bfd2d28-7ed6-11e4-9d1e-beb9c65d9256.jpg)

In the above, the red lines are pillars. This is a side view, and the picture plane is the same as the plane in which the pillars lie (so it is a somewhat simple case actually, since they do not have to be parallel).The green lines show the boundaries of a cell that comes out of the picture plane, and the blue lines are a cell going into the whiteboard. Their intersection is the black-hatched area, which has the 6 red dots as its vertices.