Add data simulator

[andrewtavis]

Adding a data simulator would be a positive addition to causeinfer in that it would allow users to more accurately check general accuracies and compare models.

Files to create:

• causeinfer.data.simulation.py
• examples/simulation.ipynb
• Adding the dataset to examples/an_iterated_model_dataset_comparison.ipynb

The simulator would then need to be documented, which should be simple with autodoc :)