Sorting and Grouping for ft.DateOffset with Multiple Units

#### Bug/Feature Request Description
For group_features() in feature_set.py, features are sorted then grouped using a key function. Within this key function, _get_use_previous appears as one of the factors to sort and group features.

In the implementation shown in #732, _get_use_previous returns a tuple with the unit and value for timedeltas with one unit. However, Timedeltas with more than one parameter has an empty tuple.

#### Expected Output
For features with a Timedelta containing multiple units, group features should sort and group features with multiple units properly.