A new application, pvtu2vtu

pvtu2vtu merges VTU files of PVTU into one single VTU file. Apart from the mesh data, all property data are merged as well by using this utility.

Utility removeGhostData also merges VTU files. It uses VTK functions to merge VTU files of PVTU. However, field data such as integration point data, are not supported in PVTU (see: https://kitware.github.io/vtk-examples/site/VTKFileFormats/#punstructuredgrid), meanwhile the ghost elements are not removed from the merged vtu files.

1. Feature description was added to the changelog
2. Tests covering your feature were added? A test is included (see the merged mesh below).
3. Any new feature or behavior change was documented?