Support gmsh file format version 4

Until then, you can convert your mesh with
```
gmsh your_msh4.msh -save -format msh2 -o your_msh2..msh
```
(via https://github.com/flow123d/flow123d/issues/1220)

It's fairly different from the currently supported v2: https://gmsh.info/doc/texinfo/gmsh.html#MSH-file-format