Add generic column split-context helpers

[munoztd0]

Generic utilities from junco

• cur_col_split_path(.spl_context)
• in_column(col_path, .spl_context)

Made by @wwojciech

Current implementation and tests are available in junco PR #423.

[wwojciech]

@gmbecker - I am now thinking about considering support for regular expressions in cur_col_split_path(). This is because, when there is a col-split (say, by the "ARM" column variable) and the split_fun is add_overall_col() or add_overall_level(), the current column path for the columns appended by these functions has ARM as a split variable. So, if there is a need to distinguish whether the current col-split is within the actual levels of ARM or within the artificial levels created by the two functions mentioned above, the generic utilities from junco are not very helpful.