ML Classes management

We should have a command arkindex classes <corpus_id> <path/to/db.csv> that allows us to manage ML classes in a corpus:

• the csv file is a simple 2 columns db using the ID and the name
• list all existing classes through API
• add missing classes from CSV and store their ID
• delete classes not present in CSV
• update classes where name do not match
• store final state in CSV