* 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
+ cleans up Stan model compilation by switching over to the rstantools-based workflow (see https://mc-stan.org/rstantools/articles/minimal-rstan-package.html for more info)
+ minor documentation change: {devtools} -> {remotes}, which is better for end-users
+ adds RStudio project file which makes it easier for community to get started with contributing to the package
