An option for the form of the `interpreted_answer` note for MCQ parts

[christianp]

At the moment, the interpreted_answer value is just the 2D array of booleans used by the marking algorithm.

For "choose one from a list" parts, it'd be nice to have the index of the student's choice, as an integer.

For "choose several from a list", it'd be nice to have a list of choices the student ticked.

For "match choices with answers" with "one from each row", it'd be nice to have the index of the student's choice for each row.