Add logic to compute scores

[nbars]

If there are exercises that are scored, we need logic that processes the data so that we can build, e.g., a scoreboard based on it.

We need these functions (besides maybe others):

• A function to get the best score for a specific exercise/task/user (or user group combination).
• Some function to get the highest and the lowest score for each task of each exercise. These values are needed to, e.g., normalize scores.
• Some way to provide per exercise (task?) formula to map the score values to a desired range.