Add reader for VTK files with vertex data
What does this MR do?
Create a reader for VTK output files of this project. It is only able to read VTK unstructured grids.
The reader requires the
vtk Python library. Since it is not yet available from PyPI for Python 3.9, we install it from source into the Docker environment image.
This MR also adds tests for the VTK reader and a separate CI job for Python builds.