A function which aids the construction for users to create their own split function for combined columns
Arguments
- nm
character(1). Name/virtual 'value' for the new facet
- label
character(1). label for the new facet
- levels
character or NULL. The levels to combine into the new facet, or NULL, indicating the facet should include all incoming data.
- rm_other_facets
logical(1). Should facets other than the newly created one be removed. Defaults to `TRUE`