GammaGammaModel API Improvements (#758)

* utils.customer_lifetime_value

* expected_customer_lifetime_value

* WIP clv.models.gamma_gamma.py

* gamma_gamma API

* fixed circular import

* gamma_gamma tests

* delete tests/datasets/test_summary.csv

* clv test_utils.py

* remove expected_purchases(future_t=0)

* remove monetary_value arg

* WIP docstrings

* notebooks

* docstrings

* Revert "notebooks"

This reverts commit a3154d9.

* gamma-gamma notebook

* docstrings