Grade reports should include the amount of time that the submission is late

[DavidWhitlock]

A student and I were discussing their late projects and, when we looked at the submissions, they were only a couple of hours late. I should augment the grade reports to include how late (e.g. "3 days") the submission was.

[DavidWhitlock]

Also, the reports shouldn't list a grade of "-2.0" for assignments that do not have a grade.

And the -2.0 should not be included in the sum of the points.

[DavidWhitlock]

The work for issue #377 will help prepare for addressing this issue.