TODO

store original mesh (cell_sets cannot be saved to vtu-format, so convert them to cell_data)

store bulk element ID of preferred domain for internal boundaries [Aqeel]

offer option to check 2D meshes for common surface normals (n=(P2-P1 x P3-P1), sign(n*ez or e_user) and possibly enforce it (change element connectivity)
  
try msh2vtu on further Gmsh tutorial examples

verify domain cell numbering of mixed domains (e.g. tri and quad), i.e. cell numbering in node connectivity matches with vtu-file (and thus OGS), ideally implement OOP style (node_connectivity class), possibly try to use remove_lower_dimensional_cells() for domain or copy cells/cell_data instead of dicts 

try other file formats (xdmf, ...)