Assign a zero for tests not taken during test availability window

It would be good if Populi could have the ability to enter a default "0" grade in online tests once the test period is over and student or participant has not completed the test. As it behaves now, sometimes a student has missed the test period and populi leaves the test ungraded. This creates a bit of a problem when an instructor has not noticed and then calculates the final grade for the student in the course. If the instructor does not notice that the test is ungraded, then the test will be omitted from the calculation of the final mark making the student's final grade inaccurate.