remove studip-nbgrader interface

id_generation/generate.py

id_generation/hashing.py

id_generation/mail.py

id_generation/nbgrader.py

id_generation/student_database.py

id_generation/studip.py

report_creation/create_report.py

report_creation/plotting.py

report_creation/read_csv_results.py

report_creation/report_writer.py