VTK legacy support for unstructured grids
I've added support for VTK legacy files, although unstructured grids only. This is useful because
- QGIS has vtk legacy output
- GMSH has vtk legacy output
The original plan was to generally replace gmsh's own format with vtk. However, apparently GMSH doesn't output physical groups into vtk format. So for now I've only switched geometry surface conversion to vtk-format because it definitely doesn't matter there. For general meshing via GMSH it's probably worth discussing if we should switch or not... or if there is a way to output physical groups somehow.