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

.github/workflows/tests.yml

@@ -1,7 +1,8 @@
 name: CI
 on: push
 jobs:
-  tests:
+  fast-tests:
+    name: fast (without R)
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
@@ -10,5 +11,22 @@ jobs:
         python-version: 3.8
         architecture: x64
     - run: pip install nox==2020.5.24
-    - run: pip install poetry==1.0.10
-    - run: nox
+    - run: pip install poetry==1.1.4
+    - run: nox -s format lint ci-tests-fast safety docs
+  slow-tests:
+    name: slow (with R)
+    runs-on: ubuntu-latest
+    env:
+      R_LIBS: .r_libs
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions/setup-python@v1
+      with:
+        python-version: 3.8
+        architecture: x64
+    - run: mkdir .r_libs
+    - run: sudo apt-get install r-base r-base-dev libcurl4-openssl-dev libxml2-dev patchelf
+    - run: R -e "install.packages('forecast')"
+    - run: pip install nox==2020.5.24
+    - run: pip install poetry==1.1.4
+    - run: nox -s ci-tests-slow