Average grades in export [REBORN]

[Belissimo-T]

closes #2001

TODO: Tests

this does not cover several important details:

1. evaluations from previous semesters are included

2. evaluations from other programs and course types are included

3. only published evaluations are included
   a. even if the unpublished evaluations are part of the export

4. which contributions are used

5. we are averaging over the averages of several results

6. are results with not enough voters used

So this test should be extended / more tests should be added. I think it is very easy to mess up some of these details in the future and when that happens, these tests should catch it