[doxygen,bugfix] Don't have an \ingroup command right after a \brief documentation

Otherwise doxygen doesn't recognize the end of the \brief description,
and merges everythings that comes after it.

Seems like a doxygen bug to me, but easy to work around.
[cleanup] Remove obsolete method MultiYGridSetup

It was still in use when YaspGrid constructors accepted the number of elements
per direction in a FieldVector<int>.  Now that that possibility is gone, one
MultiYGridSetup method can be removed, too.
Add method Entity<0>::count (unsigned int codim) to the implementations

Currently, the method exists, but the argument is a template argument
rather than a run-time parameter.  It was decided in

https://dune-project.org/flyspray/index.php?do=details&task_id=1455

to change this.  As a first step I introduce the method with a run-time
parameter to the grid implementations.  For the time being, both methods
can coexist peacefully.
Add method Entity<0>::count (unsigned int codim) to the grid interface

Currently, the method exists, but the argument is a template argument
rather than a run-time parameter.  It was decided in

https://dune-project.org/flyspray/index.php?do=details&task_id=1455

to change this.  A previous patch has added this method to the grid
implementations.  This patch adds the method to the interface.
This does not break third-party grid implementations that do not
implement the method, as long as this method is not actually called.