Export per-repository scores to a CSV file
export_scores.RdComputes every student repository's per-question scores from the project's
grading database and writes them to scores.csv in the project root, one
row per repository with one column per question plus a total column.
Scores are recomputed the same way mark() displays them: the points of
the selected rubric items are summed and passed through the question's
grading mode and score bounds. A repository/question pair that is not yet
graded (no selected rubric item and no public comment) is written as NA,
and a repository's total stays NA until all of its questions are
graded.