Parallelize trixi2vtk

In GitLab by @sloede on Mar 21, 2020, 22:47

This should be fairly easy, e.g., with a `@distributed` loop over all files to convert. The only part that is not inherently thread-safe is the writing of the PVD file. This could be achieved, however, by returning the resulting VTK file (currently named `vtk`) as an array `[vtk]` and then use `vcat` as the reduction operator. This should result in an array of VTK files, that can then be written serially to the PVD file.