Feat: Generate visual data model

I've added the ability to generate images out of the data model. Check the README for details. Thanks for merging :-)

README.rst

@@ -126,6 +126,6 @@ First, install GraphViz globally:
 
 Then, run following to generate an image out of the models:
 
-    (venv) $  manage.py graph_models --pydot -a -g -o steering_data_model.png
+    (venv) $ ./manage.py graph_models --pydot -g -o steering_data_model.png program profile