Average score method as alternative to sum of score on structure node

In some cases it is usefull to calculate the average score instead of the sum of score. E.g. when all assessable course elements generate the same max score (e.g. 100), the average can be a usefull rule to tell the performance of a student. 

ToDo: 

• Implement a radio single selection element in the scoring configuration of a structure node that appears when scoring is enabled
• Option 1: Sum (default)
• Option 2: Average
• Implement averageScore() Method that takes an array of course elements and calculates the average
• A course element that has no score (test not taken by student) is not taken into consideration for the average calculation, only elements shall count where the user actually has a score value