------------------
* .multiple_difference now always performs pairwise comparisons.
* Added Bayesian multiple_difference_joint methods for joint comparisons.
* Difference data frames now always return consistent column names.
* Add level_as_reference to multiple_difference methods to
provide control over the order of the difference.
* Added as_cumulative method to create models based on
a cumulative representation of the data.
* Added CI/CD to the library.