Merge branch 'feature/dune-alugrid-gmsh-parallel' into 'master'

[gridcreator] Fix gmsh grids for parallel alugrid runs

Dune-alugrid needs the gmsh reader to only fill the grid factory on rank 0.
See mergerequest !14 https://gitlab.dune-project.org/core/dune-grid/merge_requests/14.
This is different e.g. for UG that needs the grid on all processes.

See merge request !183