[Dune] GridSelector change merged.

[Robert Kloefkorn]

Dear DUNE community,

I have just merged the following GridSelector changes to the master
branch of dune-grid.

The auto generated GRIDTYPE in config.h must from now on be enabled
manually by passing the following cmake variable

-DDUNE_GRID_GRIDTYPE_SELECTOR=ON

Otherwise no predefined grid types will be available since the default
setting is OFF.

Regards,

Robert