Toggle whether all units of competency are selected when enrolling a student into a class

Currently, when enrolling a student in a class all of the units of competency against the qualification are selected by default.

With qualifications which have a large number of units, this leads to a situation where we are required to deselect more units than we would have had to select if there were no units selected by default. 

It would be great if there was a setting which would allow us to choose if all units are selected or deselected by default or if only core units are selected by default.