factoring out plotting of annotations and statistical testing into two separate functions

[webermarcolivier]

In the issue #53, @normarius raised the topic of factoring out the plotting of annotation and the computation of statistical tests into two separate functions.

• Your toolbox currently offers two principal features that are blended into one API-function: (1) Compute and format p-values, (2) and create annotated brackets. I'd suggest to factor out a function add_annotated_brackets() that is completely agnostic about any testing and p-values. This function creates the brackets and annotations according to the formatting settings. add_stat_annotation() could then just call add_annotated_brackets().
• You could also better factor out statistical testing part, such that the users can interfere with the p-values, filter the brackets of interest etc. (see e.g. Hide non-significant annotations #50), before they actually add the brackets.
• Personally, I think there are plenty of other libraries offering statistical testing. Keeping up with with all methodologies (bonferroni, bootstrapping, anova) is possibly difficult. I'd therefore put the primary focus on how to print these p-values (or other information), and make it as easy as possible for the user to create these annotations using the seaborn semantics.
• The reason for these suggestion: With the current solution, one has to adjust several default parameters to use custom annotation and to disable the testing. It took me a while figure this out.

[webermarcolivier]

Now supported in trevismd/statannotations.