mcq/gmcq: allow partly correct answer options and any correct answer option

[oliverfoster]

Subject of the issue

Currently the mcq and gmcq options are either correct or incorrect and you must pick all of the correct options.
Partial correctness can only be obtained by selecting at least one correct and at least one incorrect option in a multi-select mcq.

I would like to extend them to allow:

Partly correct answer options

It would be useful to allow an option to be partly correct. Especially for branching which currently progresses on correct, partly-correct or incorrect responses. Such that a single select mcq/gmcq can be partly correct.

Any correct answer option

It may be that an mcq has 3 options, 2 of which are correct and only 1 is selectable.

Note:

This pr does not cover cases of multi-select mcqs where from a larger number of possible correct answers a smaller number are selectable but only an even lower number are required answers. Selectable and required are equal here if selectable is smaller than possible. i.e. you must pick selectable correct answers if there are more possible correct answers. This would require an extra property defining the required answers value.