Add Pintilie Tamoxifen dataset

[andrewtavis]

This is an extensive issue that involves the creation of the following files:

• causeinfer.data.tamoxifen.py where the data will be loaded and formatted
• examples/medical_tamoxifen.ipynb where all valid models will be ran over the Pintilie Tamoxifen dataset
• Adding the dataset to examples/an_iterated_model_dataset_comparison.ipynb

The dataset can be found in the datasets directory.

causeinfer.data.tamoxifen.py would then need to be added to the documentation and be tested, but these should be easy steps.