Upgrade GMSH reader to GMSH v4

The Dune::GmshReader is not able to read Gmsh files of the newest version 4. Here is a very simple example of a square grid:

Reading this mesh file results in the error

Dune::IOError [read:/opt/dune/dune-grid/dune/grid/io/file/gmshreader.hh:297]: can only read Gmsh version 2 files

However, I was using Gmsh v3 before which was working fine.

Update the GmshReader to support more recent versions of Gmsh. From a quick glance into the Gmsh docs, I would expect that this is quite a bit of work, as the specs changed considerably. Any effort into this direction is highly appreciated! I offer my help but looking at the current reader I don't see myself capable of handling the implementation alone.
Mention the supported versions of Gmsh explicitly somewhere (doxygen, readme, ...). The supported version is only found in the error message. Setting up a more elaborate mesh file only to find that the Gmsh version used is not supported is quite frustrating.