Empty-cases-added bug fixed
Comment of fetch_word_cases_of_language modified

- Comment of fetch_word_cases_of_language modified
- Unnecessary 'print' commands removed
Performance optimization and minor corrections

Performance optimization in add_group_to_db():
g.db.commit() outside of function so all insertions are committed in one commit

New functions:
fetch_language_id
fetch_lecture_id
built in function add_group_to_db()

Correction of description in db_scheme.sql
Writing csv files to db added

new site /add/files added that writes csv files from database_csv to db