We now compute the average CBM score, accuracy, CBM bonus and enhanced
accuracy, both for the entire quiz, and for just the questions answered.
Note that these calculations must work correctly in the presence of
descriptions, ungraded questions, and manually graded questions. For
example, imagine a essay added at the end of the quiz "Summarise what
you learned attempting this exercise." This might have max mark zero or
non-zero. The CBM statistics just ignores questions like that.
