A CMake macro called `add_precice_file_links` is added. If one
calls this macro in a `CMakeLists.txt` via `add_precice_file_links()`,
file links to all XML files in the directory of the `CMakeLists.txt`
are created.

The paths in tests have been adapted accordingly to use 
`CMAKE_CURRENT_BINARY_DIR` instead to refering to the source directory.

A lot of the codes for monolithic coupling are out of date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.

- 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