function split up to access gmsh object

[jon-proximafusion]

The assembly_to_gmsh function is great

In some case we are looking for a little more control over the meshing parameters.

It would be useful if we can return the gmsh object so that the user can mesh the geometry with their own meshing parameters. mesh order

Proposal
Remove some of the gmsh commands from the function and allow the user to do these

The first part loads the geometry and does the tagging and returns the gmsh object, perhaps it does not need these lines of code

    gmsh.model.mesh.field.setAsBackgroundMesh(2)

    gmsh.model.mesh.generate(3)
    gmsh.write(mesh_path)

    gmsh.finalize()

Alternative solution (not ideal)
One option is to pass in a large amount of meshing parameters like set_size, min_mesh_size, max_mesh_size_backgroung mesh size etc
However this would be difficult to manage the number of meshing options available.

[jmwright]

Addressed in #6