Add ability to compare fields

[valentin-seitz]

This PR introduces the ability to compare produced fields. Therefore necessary:

• Introduced the generation of reference results via the generate_reference_results.py script.
• Added .gitattributes for lfs storage of reference results archives
• added correct exit status of systemtests.py

This PR solves:

• Add some machine readable file describing the versions going together to make up a reference output #353
• Adapt the tests.yaml to contain the reference files #351
• Have systemtests return right exit code for workflow #361

[MakisH]

Nice! 🚲

Works on my machine (after checking out the precice-config.xml from master).

Some questions for me to better understand, not necessarily something to change. Please ask for review again when ready.

[MakisH]

Some follow-ups are already in the project board, this can already be merged.