Convenience wrappers for global parameters and meshing

[gpeairs]

Currently, users call SolidModels.gmsh.model.mesh.generate(dim=3) to generate a mesh on dim-dimensional entities, and use SolidModels.gmsh.option.set_string or ...set_number to set various Gmsh options. Some options can also be set using MeshingParameters provided to render!, but some of these are global or mesh-only settings that one might want to change after rendering. We should define functions that wrap these and/or provide convenience functions like generate_mesh(dim=3; mesh_order=1, surface_mesh_algorithm=6, ...) for [re-]generating meshes with different settings without re-rendering.

(We could ideally also supply MeshingParameters to such a function, but we currently can't change mesh sizing information after rendering, so it might be a little confusing to supply MeshingParameters with different mesh_scale and α_default to no effect. An interesting idea would be to separate rendering and mesh size field creation by saving the meshsize_dict produced during rendering for later. This could even allow saving an unmeshed geometry .xao file alongside a mesh sizing .json to be loaded in another Julia session.)