[WIP]: Addons to dumux are stored in dumux-addon and are also installed into a dumux-addon directory

- FVCA publication code moved to https://git.iws.uni-stuttgart.de/dumux-pub/jaust2020a
- This branch contains more than just the FVCA code and cleanup.
- Added some new examples for non-smooth coupling interfaces
- Added some new examples for 3D
- Monolithic coupling depends on special DuMuX branch
- Added CI
- Added code formatting