Export gradebook

• https://nbgrader.readthedocs.io/en/0.7.x/command_line_tools/nbgrader-export.html
• https://github.com/jupyter/nbgrader/blob/v0.6.1/nbgrader/docs/source/user_guide/extract_grades.py
• Example: nbgrader export --db="sqlite:////tmp/gradebook.db" --to pggrades.csv