Add module for causal discovery

[Zethson]

We recently implemented causal inference with dowhy. However, currently ehrapy has no module to also conduct causal discovery or in other words determine causal graphs on its own. It'd be great to have a robust API for causal discovery which, just like dowhy, tries to robustly infer causal graphs. Robustness is very important here.

• Research on which Python packages are out there
• Papers that used them
• API design draft
• Implementation + tests
• Tutorials