MDL-32188 question CBM: alter score handling.

We now change so that minfraction is -6 and maxfraction is 3, so getting
the question right a low certainty gives maxmark marks, and you get a
bonus for being more confident (rather than being penalised for being
unconfident). Mathematically it is the same, but the difference is
importnat psychologically.

We also change how partially correct scores are handled.
It is too harsh to penalise a partially correct score with full
certainty by doing a linear interpolation between -6 and +3. Instead,
any partially correct score (e.g. 0.5) becomes that fraction of the
correct score (e.g. 0.5 * 3 = 1.5). Also, any incorrect score is treated
as 0, so if you have a multiple choice question that normally gives a
negative score for a wrong choice, this will now never give a score of
less than -6.

Finally we change how this is displayed to students beside the question.
Rather than saying "Marked out of 1.00", we say "Base mark 1.00", and
then later we say "CBM mark 3.00" (or whatever it is).
MDL-32188 question CBM: improve display of the certainty choices

Switch to the neutral C=1, 2, 3 mode. Also add a helpicon, and improve
line-wrapping.
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.