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.

Tests run with DuMuX: 3.4, 3.5 and on nightly with DuMuX master
Tests run with preCICE: 2.3.0 and 2.4.0

A lot of the codes for monolithic coupling are out of date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

- Some basic API functions for communicating vector quantities 
  were added.
- A DuMuX dummy solver for testing the new and old API functions 
  were added.

The `couplingadapter.cc` was missing the include of `limits` in
order to use `numeric_limits`. See issue #27.

This configures the mkdocs documentation to use the `mkdocs-material` theme
and sets the colors to orange and blue.

This adds:
- a freshly initialized mkdocs configuration in `doc/user`
- a ReadTheDocs configuration in the root of the repository
  named `.readthedocs.yaml`

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

We use prebuilt Docker images on the `ajaust` namespace and push them to DockerHub. This commit makes sure that the CI is using these new container images. These changes have been done while moving the CI to a new VM.

Related changes:

* Tolerance of partitioned tests was increased to `5e-5` as the results changed slightly using the new Docker images on the new VM.
* Added a copy of `fuzzycomparevtu.py` from the DuMuX repository into this repository. This allows us to be more independent of the location of the DuMuX repository.

Previously private member variables of the CouplingAdapter and related
classes were prefixed with an underscore which is illegal. The private
member are now suffixed with an underscore.

The documentation was out of date and not everything would have been interesting for user anyway.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.

The changes of merge request !18 were missing in the changelog.

- 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

Changes on DuMuX master may be incompatible with previous releases. We
check against DuMuX master in the CI to be notified of such changes.
However, chacking against DuMuX master also leads failing nightly tests
often which give the impression that the released versions of the
adapter are broken even if this is not the case.

This change allows checks on DuMuX master to fail. A warning will be
created in these cases instead of an error.

A changelog is added to document changes that are made to the adapter.
Information on the initial release (v0.1) is incomplete.

Fix ci to use dumux master

See merge request !13

Fix CI and improve compatibility with DuMuX 3.4

Closes #22 and #17

See merge request !12