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

src/urban_meal_delivery/init_r.py

@@ -0,0 +1,28 @@
+"""Initialize the R dependencies.
+
+The purpose of this module is to import all the R packages that are installed
+into a sub-folder (see `config.R_LIBS_PATH`) in the project's root directory.
+
+The Jupyter notebook "research/r_dependencies.ipynb" can be used to install all
+R dependencies on a Ubuntu/Debian based system.
+"""
+
+from rpy2.rinterface_lib import callbacks as rcallbacks
+from rpy2.robjects import packages as rpackages
+
+
+# Suppress R's messages to stdout and stderr.
+# Source: https://stackoverflow.com/a/63220287
+rcallbacks.consolewrite_print = lambda msg: None  # pragma: no cover
+rcallbacks.consolewrite_warnerror = lambda msg: None  # pragma: no cover
+
+
+# For clarity and convenience, re-raise the error that results from missing R
+# dependencies with clearer instructions as to how to deal with it.
+try:  # noqa:WPS229
+    rpackages.importr('forecast')
+    rpackages.importr('zoo')
+
+except rpackages.PackageNotInstalledError:  # pragma: no cover
+    msg = 'See the "research/r_dependencies.ipynb" notebook!'
+    raise rpackages.PackageNotInstalledError(msg) from None