Upgrade GMSH reader to GMSH v4

Description

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:

• square.msh
• square.geo

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.

Proposals

• 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.