gmsh2dgf exists multiple times

The target gmsh2dgf exists in dune-alugrid and in dune-grid. The version in alugrid does not only generate a dgf file, but also a vtu file. The problem with this doubling is, that one cannot build dune-grid and dune-alugrid in one big compile-step together, e.g. using a cmake super-build. The gmsh2dgf would then exists multiple times and a cmake error is raised.

I suggest to rename one of the targets, e.g. the dune-alugrid target into gmsh2other or gmsh2dgfplus. Or just move the dune-alugrid version into dune-grid.