From DFN generation in PorePy to reactive transport modeling in OGS

This merge request is for generating a Discrete Fracture Network (DFN) using Porepy for the DFN workshop. It was initially started by @nagelt Thomas Nagel.

The DFN generated in Porepy is exported as a VTU file. After some cleanup using OGS tools, the file is ready to be used in an OGS simulation, specifically for the HC process.