Add option to export VTU files from unstructured meshes, and remove dependency on vtk

[rherrero-pf]

Description

At the moment we export unstructured meshes in legacy VTK format. Ideally, one would be able to export VTU, which is a modern version and provides better compression.

https://discourse.paraview.org/t/vtu-or-vtk-file-formats-which-is-preferred/6976/2

Alternatives

Continue using .vtk files, and transform to .vtu in paraview

Compatibility

The enhancement should still allow export meshes in legacy VTK using the default behavior.