This can help interpreting the ci logic once we add pipelines for more 
events.

The idea is to evaluate differences between the current branch and the 
target branch for merge requests. Having a default we don't notice if 
this breaks when someone removes the target branch variable passed from 
the parent pipeline.

since we now run the test selection stage in the same container as the
build state, this is no longer necessary

also, do not only check the current project headers, as downstream
modules may want to test against changes in dumux

the ci seems to sometimes fail to pass the created .yml file to the
subsequent stage. Thus, we now always run the test-selection stage but
skip script execution in case we are not in a merge request

use the generator script from the template instead