implement contrasts

[sreichl]

consider (again) how to implement contrasts

• Consider changing the whole module to a one-datasets setup, like in spilterlize_integrat. Then the contrasts, and all other configs, could be integrated into the config file. Con: one workflow instance/module load per analysis. Q: How often do we analyze each split?
• For the README, add code of improved examples for contrasts based on what I've recently done. This will help everyone without needing extra resources. Complement the contrast matrix, which is hard to understand, with a variable case.
• Think about adding contrasts with an additional column in the annotation file. Possible?