[cmake][coverage] do not use pretty-xml

See merge request tools/frackit!192

[ci] require manual start of pipeline in mrs

See merge request tools/frackit!191

Most of the time, changes to the docu will occur in header files, thus,
it should be checked not only if the doxygen setup in doc/ has changed.

This avoids the triggering of many pipelines during a phase of
development with an open merge request. For commits to master and for
tag commits, pipelines are still automatically started.

Fix/ci code coverage

See merge request tools/frackit!190

Feature/coverage reports

See merge request tools/frackit!188

Fix/ci build doc job

See merge request tools/frackit!185

[docker] add Dockerfile

See merge request tools/frackit!182

This ensures that not all artifacts are downloaded

We allow for passing the number of entities to be created via the
command line, such that this can be used in the test suite to speedup
computations.

This turns examples into "tests". We actually want to have them tested
in the ci as well. Moreover, the add_application macro was basically the
same as add_test, just not adding an actual test.

Feature/docu improve installation notes

See merge request tools/frackit!180

[ci] use default image and specify only mrs for all jobs

See merge request tools/frackit!181