Create a mesh reusing mat_ids and descs of mesh_in.

def make_mesh( coor, ngroups, conns, mesh_in ):
    """Create a mesh reusing mat_ids and descs of mesh_in."""
    mat_ids = []
    for ii, conn in enumerate( conns ):
        mat_id = nm.empty( (conn.shape[0],), dtype = nm.int32 )
        mat_id.fill( mesh_in.mat_ids[ii][0] )
        mat_ids.append( mat_id )
        
    mesh_out = Mesh.from_data( 'merged mesh', coor, ngroups, conns,
                               mat_ids, mesh_in.descs )
    return mesh_out