This is a simple initial test of using gmsh to create a mesh to use with
PyLith.  At present, there is just a single example, mesh_tet4.geo.  This
example includes the geometry file (geometry.geo).

If used in interactive mode:
1.  Open the mesh_tet4.geo file from within gmsh.
2.  Mesh the volumes using the Mesh->3D command.
3.  Optimize the mesh using the Mesh->Optimize 3D command.
4.  Export the mesh using either the File->Save command (uses default gmsh
    format), or the File->Save As command (select output type from list).

If used in command-line mode, you can just run the run_gmsh.sh script, which
creates a mesh in gmsh format.

At present, I haven't figured out how to do the equivalent of nodesets, or
how to do logical operations on physical groups.

At present, it appears that the simplest method of producing various
output formats is to do it in interactive mode.
