Only run ci on merge requests

Our current CI tests take quite some time to run, especially a full experiment test run. There are also only two runners for this repo available which are at the same time also busy with stuff in the codec repo, often one of them is doing a long task there and is not available here.

Proposal would be to only run the tests on merge request pipelines and not on push pipelines as we do in the codec repo already. If you want to test during development please run the tests locally and only open a merge request when you think you are done.