Merge branch 'feature/gmsh-reader-boundarysegments-1d-grids' into 'master'

ref:core/dune-grid 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]
-   Adds a test for 1d gmsh boundary data with OneDGrid

See merge request [!348]

  [#93]: gitlab.dune-project.org/NoneNone/issues/93
  [!348]: gitlab.dune-project.org/core/dune-grid/merge_requests/348


Closes #93