To solve this [issue](https://github.com/facebook/prophet/issues/1437) by @chongdae
Korean lunar calendar which is different from Chinese lunar calendar is added python-holidays package recently.
This commit is about dropping the Korean and KR class in hdays.py file and hdays_part1(holidays) will work for Korean holidays.
```
import fbprophet.hdays as hdays_part2
import holidays as hdays_part1
``
* 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
* Add functions for serializing to/from JSON
* Fix list vs. series type issue, track version
* Avoid DateTimeIndex
* bugfix
* another fix
* Fix copy test
* Fix issue with pre-epoch dates
* Handle empty datetime series
* modified cross_validation to allow custom cutoffs
* moved set period, initials and identify larg. seas
* modified the diagnostics and added the test
* reverted cv default value tests and added a new custom cutoff test
* reorganized to raise the seasonality period warning message even if cutoffs are manually specified
* moved the initials vs. seasonality check
* changed assertCountEqual to assertItemsEqual in cv
* modified to test lengths instread of cutoff values
Co-authored-by: Fusi Marco <Marco.Fusi@valuelab.it>
* sampling iters arg name, logic changes
* Bump cmdstanpy version in requirements to 0.9.5
* Change Model to CmdStanModel
Co-authored-by: Ben Letham <bletham@gmail.com>
* Add multiprocessing Pool and create function for single cutoff forecast
* add params to single forecast function
* Add iterable for input params for pool
* Add docstring for single cutoff forecast func
* Add check for multiprocessing in test_cross_validation
* check ofr is None and is True and add better description for multiprocess in docstring
* Raise error if wrong args chosen and add test
* fix conflicts
* Change arg to True/False, model.kwargs
* docstring units and few more fixes
* change to iterator and add back tqdm to for loop
* add option in diagnosics notebook about multiprocessing option
* add extra test for checking calls to single forecast func
* tqdm
* Added progress bar to the crossvalidation
In order to improve the user experiance a progress bar is added to the crossvalidation loop.
* Update requirements.txt
* Update python/fbprophet/diagnostics.py
* updated further
* Update requirements.txt
* 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
* update holidays dependency according to the new structure
* fix backward compatibility problem
* by-pass version dependency for holidays package
Co-authored-by: Ben Letham <bletham@gmail.com>
* Style modifications to fbprophet
* fixing cases where tests were broken
* minor style changes
* changed f-strings to named formatted strings due to Python2 build
* fixing syntax errors
* switching to formatted string quoting using {!r}
* Fixing ValueError Styling in my own branch
* fixing type and changing back one quoted string
