Include element halos in dg-ify fields process in vtk_write_fields

Currently if you pass discontinuous fields to `vtk_write_fields` with a continuous `model` mesh  then it will convert the continuous mesh into a discontinuous mesh. In doing so it does not currently copy over the `element_halos` which are required to write the vtkGhostLevels to the pvtu file. This commit copies the element halos to the discontinuous mesh, if required.