Implement tidy_group_by()

[larmarange]

cf. ddsjoberg/gtsummary#1540

on broom.helpers side

New function tidy_group_by() and new option group_by in tidy_plus_plus().

This function will be in charge of preparing the results and, if needed, returning an additional group_by column in the results.

By default (group_by = "auto"), if there is a y.level column and this column has a minimum of 2 different values, then group_by will be populated with this column, or, if there is a component column and this column has a minimum of 2 different values, then group_by will be populated with component, otherwise no grouping will be performed. It will cover most standard use cases.

group_by = NULL to force no grouping.

group_by = "effect" (or another column name) indicating that we want to group according to a specific column returned by the tidier.

An option group_labels could be implemented to allow the user to rename the different groups.

Such an approach should be pretty flexible. It even allows the development of custom tidiers with other grouping options.