Score systems issues

[Douglas Cerna]

I found these while I was working on the gradebook performance task. Just to have them on record:

1. what type is the passing score in the 'letter grade', 'pass/fail' and 'extended letter grade' score systems. minimum passing type, maximum? can this information be inferred from the index view of the score system? i couldn't. maybe we should explicitly put it in the index view.

2. discrete values score systems have a ._isMaxPassingScore, but ranged values score systems don't and i think they should, maybe set to True? the score validation view needs this. right now it thinks that B, C, D, F are extracredit and i don't want to use .providedBy(...) checks

3. the _isMaxPassingScore property is forbidden for scoresystems (no security declaration) and there's no an API to get to it.

4. For the '100 points' score system, fromUnicode('0') raises no exception. also if you set 0 on the Score student form, you get a kind of confussing message "Constraint not satisfied".