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

tests/test_init_r.py

@@ -0,0 +1,19 @@
+"""Verify that the R packages are installed correctly."""
+
+import pytest
+
+
+@pytest.mark.r
+def test_r_packages_installed():
+    """Import the `urban_meal_delivery.init_r` module.
+
+    Doing this raises a `PackageNotInstalledError` if the
+    mentioned R packages are not importable.
+
+    They must be installed externally. That happens either
+    in the "research/r_dependencies.ipynb" notebook or
+    in the GitHub Actions CI.
+    """
+    from urban_meal_delivery import init_r  # noqa:WPS433
+
+    assert init_r is not None