Added two types of grading schemes (in addition to "exact") to ordering exercises:
- "position": count elements as correct, when these are on the correct position
- "relative": count elements as correct, if the neighboring element is correctly before the actual element
The results are adjusted by the same guessing correction as in the "ggw" scheme for MC exercises.
Example:
- desired order: 1,2,3,4
- provided answer: 3,1,2,4
- scheme "exact": 0%
- scheme "position": 0 0 0 1
- scheme "relative": 0 1 1 (correctly ordered 1<2 and 2<4)
A minor refactoring was also performed to ease code reuse.
