MDL-32188 question CBM: compute summary stats for CBM behaviours

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.
MDL-32188 question CBM: minimal handling of certainty -1

Certainty -1 has never been used in standard Moodle, but is
used in Tony-Gardiner Medwin's patches to mean 'No idea' which
we intend to implement: MDL-42077. In the mean time, these changes
avoid errors for people who have used TGM's patches.
MDL-32188 question CBM: use short certainty name in response summaries.