The WriteVTK wrapper: Multiblockfiles and VTKFileCollection

Before the introduction of the WriteVTK wrapper in #692 , it was easier to work with multiblock files and paraview collection files (to save multiple time steps). From one of my packages:

```
pvd = paraview_collection(filename)
...
vtmfile = WriteVTK.vtk_multiblock(filename)
for part in parts
vtkfile = vtk_grid(partname, part) #e.g special output for IGA or Immersed grids
WriteVTK.multiblock_add_block(vtmfile, vtkfile)
...
end
vtk_save(vtmfile)
pvd[state.t] = vtmfile
```

This has changed now:

1. The creation of paraview collection files has changed. Now they require a grid/dofhadler as an argument for the [constructor](https://github.com/Ferrite-FEM/Ferrite.jl/blob/2afbd6a2f0cba65070ff6c8453bde4c7a4f89995/src/Export/VTK.jl#L74) (`VTKFileCollection(name, grid_or_dh)`), which is not always known at the construction of the paraview collection. We should either make it possible to have `grid_or_dh===nothing` or remove it completely (depending on if we only want to have a thin wrapper or extend it with our own functionality).
2. We dont have any wrappers for multiblockfiles atm.