Add gmsh .msh importer

[EmJay276]

Framework for importing sections, an import function for each section is necessary.

Working on it :)

[odedstein]

Thanks, this looks like a very useful feature.
Looking at our existing function, read_mesh, do you think this should be a separate function read_msh, or this should be a part of read_mesh?

[EmJay276]

Not sure yet, the return values are completely different if I remember correctly.
I will have a look at it if I have some spare time :)