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_config.py

@@ -29,6 +29,9 @@ def test_database_uri_set(env, monkeypatch):
     monkeypatch.setattr(configuration.ProductionConfig, 'DATABASE_URI', uri)
     monkeypatch.setattr(configuration.TestingConfig, 'DATABASE_URI', uri)
 
+    # Prevent that a warning is emitted for a missing R_LIBS_PATH.
+    monkeypatch.setattr(configuration.Config, 'R_LIBS_PATH', '.cache/r_libs')
+
     with pytest.warns(None) as record:
         configuration.make_config(env)
 
@@ -43,6 +46,9 @@ def test_no_database_uri_set_with_testing_env_var(env, monkeypatch):
 
     monkeypatch.setenv('TESTING', 'true')
 
+    # Prevent that a warning is emitted for a missing R_LIBS_PATH.
+    monkeypatch.setattr(configuration.Config, 'R_LIBS_PATH', '.cache/r_libs')
+
     with pytest.warns(None) as record:
         configuration.make_config(env)
 
@@ -57,10 +63,64 @@ def test_no_database_uri_set_without_testing_env_var(env, monkeypatch):
 
     monkeypatch.delenv('TESTING', raising=False)
 
+    # Prevent that a warning is emitted for a missing R_LIBS_PATH.
+    monkeypatch.setattr(configuration.Config, 'R_LIBS_PATH', '.cache/r_libs')
+
     with pytest.warns(UserWarning, match='no DATABASE_URI'):
         configuration.make_config(env)
 
 
+@pytest.mark.parametrize('env', envs)
+def test_r_libs_path_set(env, monkeypatch):
+    """Package does NOT emit a warning if R_LIBS is set in the environment."""
+    monkeypatch.setattr(configuration.Config, 'R_LIBS_PATH', '.cache/r_libs')
+
+    # Prevent that a warning is emitted for a missing DATABASE_URI.
+    uri = 'postgresql://user:password@localhost/db'
+    monkeypatch.setattr(configuration.ProductionConfig, 'DATABASE_URI', uri)
+
+    with pytest.warns(None) as record:
+        configuration.make_config(env)
+
+    assert len(record) == 0  # noqa:WPS441,WPS507
+
+
+@pytest.mark.parametrize('env', envs)
+def test_no_r_libs_path_set_with_testing_env_var(env, monkeypatch):
+    """Package emits a warning if no R_LIBS is set in the environment ...
+
+    ... when not testing.
+    """
+    monkeypatch.setattr(configuration.Config, 'R_LIBS_PATH', None)
+    monkeypatch.setenv('TESTING', 'true')
+
+    # Prevent that a warning is emitted for a missing DATABASE_URI.
+    uri = 'postgresql://user:password@localhost/db'
+    monkeypatch.setattr(configuration.ProductionConfig, 'DATABASE_URI', uri)
+
+    with pytest.warns(None) as record:
+        configuration.make_config(env)
+
+    assert len(record) == 0  # noqa:WPS441,WPS507
+
+
+@pytest.mark.parametrize('env', envs)
+def test_no_r_libs_path_set_without_testing_env_var(env, monkeypatch):
+    """Package emits a warning if no R_LIBS is set in the environment ...
+
+    ... when not testing.
+    """
+    monkeypatch.setattr(configuration.Config, 'R_LIBS_PATH', None)
+    monkeypatch.delenv('TESTING', raising=False)
+
+    # Prevent that a warning is emitted for a missing DATABASE_URI.
+    uri = 'postgresql://user:password@localhost/db'
+    monkeypatch.setattr(configuration.ProductionConfig, 'DATABASE_URI', uri)
+
+    with pytest.warns(UserWarning, match='no R_LIBS'):
+        configuration.make_config(env)
+
+
 def test_random_testing_schema():
     """CLEAN_SCHEMA is randomized if not set explicitly."""
     result = configuration.random_schema_name()