Rename old modal_attribute with _dt designator because it uses data.table as its approach. Added a modal_test case to ensure that new method is equivalent. Added new modal_attribute function that uses dplyr.
Rename old modal_attribute with _dt designator because it uses data.table as its approach. Added a modal_test case to ensure that new method is equivalent. Added new modal_attribute function that uses dplyr.

jsonbecker/PPSDCollegeReadiness