* added `flat_growth_init()` function
* added validation for 'flat'
* changed `fit.prophet()` to handle `growth='flat'`
* added `trend='flat'` capabilities to `sample_predictive_trend()` and `fit.prophet()`
* updated STAN code to handle flat trend
* [Syntax fix] Removed unnecessary bracket
* updated documentation
* undid formatting that was accidentally applied by autoformatter
* undid more formatting that was accidentally applied by autoformatter
* added tests
* typo in `sample_predictive_trend()`
* updated notebook with example in R
* updated documentation
* add test and initial function for mdape in R
* Add separate rolling_median_func and tests
* Modify rolling median function
* fix syntax in rolling median function
* sort by h
* R/diagnostics.R
* update .rd docs and notebook
* Add mdape to performance metrics params docstring
