[bugfix] Fixes usage of typename outside of template.

With G++4.4 I got:

In file included from /home/mblatt/src/dune/current/dune-grid/dune/grid/uggrid.hh:50,
                 from /home/mblatt/src/dune/current/dune-grid/dune/grid/common/test/mcmgmappertest.cc:15:
/home/mblatt/src/dune/current/dune-grid/dune/grid/uggrid/ugwrapper.hh: In static member function ‘static void Dune::UG_NS<2>::Corner_Coordinates(const UG::D2::vector*, double**)’:
/home/mblatt/src/dune/current/dune-grid/dune/grid/uggrid/ugwrapper.hh:431: error: using ‘typename’ outside of template
In file included from /home/mblatt/src/dune/current/dune-grid/dune/grid/uggrid.hh:80,
                 from /home/mblatt/src/dune/current/dune-grid/dune/grid/common/test/mcmgmappertest.cc:15:

Indeed, there should be no typename here. This commit fixes this.
[bugfix] Removes usage of unsupported constexpr.

gnuplottest used a constexpr for no obvious reason. As this broke compilation
with g++-4.4 this commit uses a const int instead.