Update BetaGeoModel API (#709)

* _extract_predictive_variables util

* deprecation warnings

* expected_purchases_new_customer

* TestBetaGeoModel.setup_class

* test cleanup and test_expected_purchases_new_customer

* expected_probability_alive

* expected_purchases

* TODOs and docstrings

* update runslow tests rtol for new test dataset

* prob_alive_matrix plot fix and notebook testing

* alive loop in bgnbd nb

* quickstart nb fix

* docstring and TODO revisions

* docstring syntax

* docstring edits

* more docstring fixes

* docstrings indent

* docstring indent, clear codecov bug

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Co-authored-by: Juan Orduz <[email protected]>