interactVariables: Create a variable by interacting categorical variables
Description
interactVariables takes two or more variables and creates a new one that
is the cartesian product expansion of their unique values. For example, if
we cross ethnicity (with 2 categories) and race (with 4 categories), the new
variable would have 8 valid categories (e.g. black:hispanic, white:hispanic,
black:non-hispanic, etc.) and 7 categories where at least one of the
variables is missing (e.g. white:No Data).