Feature/pore network model

This MR implements pore-network models in Dumux.

TODO

• remove deprecated warnings
• discuss folder structure
• discuss caching and transmissibility laws
• discuss grid data
• discuss tests
• check docu
• clean up
• use script in bin/util/create_cmakelists.py to create CMakeLists.txt files
• change license headers to GPL v3
• put flux-related header into dumux/flux
• revise and check ps-Sw curve
• discuss and find a solution for the 2p and 2pni tests, which are very sensitive to the chosen regularization method and the parameter Swmax.
• Add description in changelog (+ backwards-compatibility policy)