[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.
> I assume that there are 3 faces for both the blue and the green cell on the side that lies on the whiteboard.

There's no reason to assume that.  When crossing a fault there can be **many** more connections than you'd intuitively think.  It is theoretically possible, although I've never seen such a model in practice, for a cell in a column on one side of a fault to connect to **every** cell in the column on the other side of the fault.  In other words, if there are `Nz` layers in the model, then there might be up to `Nz` connections (intersections) on the face that couples the cell to the fault.