Add rpy2 to the dependencies

- add a Jupyter notebook that allows to install all project-external
  dependencies regarding R and R packages
- adjust the GitHub Action workflow to also install R and the R packages
  used within the project
- add a `init_r` module that initializes all R packages globally
  once the `urban_meal_delivery` package is imported

setup.cfg

@@ -255,6 +255,8 @@ ignore_missing_imports = true
 ignore_missing_imports = true
 [mypy-pytest]
 ignore_missing_imports = true
+[mypy-rpy2.*]
+ignore_missing_imports = true
 [mypy-sqlalchemy.*]
 ignore_missing_imports = true
 [mypy-utm.*]
@@ -269,5 +271,6 @@ console_output_style = count
 env =
     TESTING=true
 markers =
-    db: tests touching the database
-    e2e: non-db integration tests
+    db: (integration) tests touching the database
+    e2e: non-db and non-r integration tests
+    r: (integration) tests using rpy2