Prophet

* Built-in json serialization
* Added "flat" growth option
* Bugfixes related to holidays and pandas
* Plotting improvements
* Improvements in cross validation, such as parallelization and directly specifying cutoffs

- Fix bugs related to upstream changes in `holidays` and `pandas` packages.
- Compile model during first use, not during install (to comply with CRAN policy)
- `cmdstanpy` backend now available in Python

* Conditional seasonalities
* Improved cross validation estimates
* Plotly plot in python
* Bugfixes

- Added [built-in holidays](https://facebook.github.io/prophet/docs/seasonality,_holiday_effects,_and_regressors.html#built-in-country-holidays)
- Bugfixes

- Multiplicative seasonality
- Cross validation error metrics and visualizations
- Parameter to set range of potential changepoints
- Unified Stan model for both trend types
- Improved future trend uncertainty for sub-daily data
- Bugfixes

* Bugfixes