Where can I find documentation on the following command?
In particular I need to know the format in which arguments are passed to custom_problem_grader, in further particular how the %evaluated_answers are indexed.
I don't think there is a good documentation for this feature. The best I can do is to point you to the two "built in" problem_graders. There is some documentation in the comments:
the avg_problem_grader is a few lines below the std_problem_grader referenced above.