A lot of the codes for monolithic coupling are out of date
and make maintenance tedious. This commit removes the monolithic
coupling since the monolithic coupling is already implemented
in DuMuX and we do not need the adapted version of the monolithic
coupling here anymore.

The subdirectory `docker/` contains the Docker recipe to build
a container containing the adapter's dependencies.

This adds configuration files for markdownlint, reformats markdown files
and also updated the configuration of the CI.

The documentation was out of date and not everything would have been interesting for user anyway. Therefore, we remove this documentation for the moment. This also adds some information about the documentation in the main `README.md` of the repository.

- This MR removes unused test cases and separates stronger
between tests and example cases. Test cases and examples 
are classified as `monolithic` or `partitioned`.
- The tests have been adapted such that the build target
`build_tests` builds all necessary executables. This means
on can now run `make build_tests` from `build-cmake` and 
afterwards run all tests `ctest` to test the adapter.
- The CI has been adapted to make use of the new build 
process for tests.

For more information check out merge request: !18

- 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