Lifetimes

- Added a data generation method, `generate_new_data` to `BetaGeoBetaBinomFitter`. zscore
- Fixed a bug in `summary_data_from_transaction_data` that was casting values to `int` prematurely. This was solved by including a new param `freq_multiplier` to be used to scale the resulting durations. See 100 for the original issue. aprotopopov
- Performance and bug fixes in `utils.expected_cumulative_transactions`. aprotopopov
- Fixed a bug in `utils.calculate_alive_path` that was causing a difference in values compared to `summary_from_transaction_data`. DaniGate

- fixed many of the numpy warnings as the result of fitting
- added optional `initial_params` to all models
- Added `conditional_probability_of_n_purchases_up_to_time` to `ParetoNBDFitter`
- Fixed a bug in `expected_cumulative_transactions` and `plot_cumulative_transactions`

- adding new `save_model` and `load_model` functions to all fitters. This will save the model locally as a pickle file.
- `observation_period_end` in `summary_data_from_transaction_data` and `calibration_and_holdout_data` now defaults to the max date in the dataset, instead of current time.
- improved stability of estimators.
- improve Runtime warnings.
- All fitters are now in a local file. This doesn't change the API however.