Creating a simple integration test suite

[batukav]

At the moment we don't have an integration test running on github. I think we should create a simple test suite which verifies after each commit that the databank functionalities are still working as expected.

I suggest that we choose a small set of trajectories, extract a few frames and calculate all the observables that the databank does. Then we can upload these trajectories to Zenodo and run the standard databank workflows whenever there's a new commit. If the new and old code give the exact same results, we can proceed with merging the pull requests.

If this sounds reasonable, we can select a single trajectory from each system/engine we support.