[gmsh] Insert boundary segments for 1d grids

Boundary segments were not inserted for 1d grids (where boundary segments are single nodes). However this is needed to e.g. attach boundary data to a Dune::FoamGrid<1, 3> or Dune::OneDGrid.

• Fixes #93 (closed)
• Adds a test for 1d gmsh boundary data with OneDGrid