Extend randomization of multiple alternatives for cases, where one alternative must be "true"
Certain evaluation schemes for multiple choice questions require that at least one alternative must be true. Otherwise, a division by 0 might appear. This is an issue, when a subset of alternatives is used (e.g. 3 out of 10 alternatives). The version know evaluation schemes with such a requirement and makes sure that in the resulting set, at least one correct alternative is correct. Note: this change does not make sure that the full set of alternatives contains at least one correct alternative.