* add implementation for constant trend
* force k and delta params to be 0s
* add tests and fix n_changepoints, changepoints_t to 0
* Add test for cv with constant trend
* Add docs and test for checking invalid input
* make changes to stan
* add transformed params block in stan and output flat trend vector
* correct syntax
* transformed params syntax
* Fix test and port changes to win stan file
* add test for flat trend function
* Add separate function for flat trend init
* fix test
* changes
* added actual tests for fit method
* precision
* syntax
* sampling not working
* sampling seems to work
* sampling not working again
* sampling works, tests to be removed
* replaced data with rmse
* replace pystan with cmdstanpy
* cleanup
* cleanup
* test for newton
* added support for multiple backends
* minor fixes
* fixed comment
* added support for --test-slow flag
* fixed import
* reverted style change
* specify backend based on env variable
* fixes
* PR fixes
* Added feature to mask seasonality
* Added conditional seasonality tests and fixed plotting it
* Fixed diagnostics for conditional seasonality and cleaned up code
* Allow both both hoidays and append holidays; match holidays in predict and fit
* Allow both both hoidays and append holidays; match holidays in predict and fit
* Add test for append_holiday features; minor fixes
* Add column name validation for append_holidays names; allow only one country
* Update memory requirement description per #326
* Fix R warning with extra regressor; disallow constant extra regressors.
* Fix unit test broken in new pandas
* Fix diagnostics unit tests for new pandas
* Fix copy with extra seasonalities / regressors Py
* Fix copy with extra seasonalities / regressors R
* Fix weekly_start and yearly_start in R plot_components
* Fix plotting in pandas 0.21 by using pydatetime instead of numpy
* Resolve conflict
* Change comments and add error column to output DataFrame
* Change file structure
* Update
* Modified diagnostics
* Update diagnostics.py following the advice on Github
* Add tests and documentation
* Change copy method into Prophet class and reflect comments
