Can add / remove files to/from existing submissions

• depot: show info or warning when "Envoyer" pressed

• modification: at least one doc must be kept; cannot proceed if documents have the same name

• tests: test_load.py -> test_Submission.py with more tests

• Submission: request_id, creation_date, modification_date in SubmissionBase with default values; request_id included in output_dir automatically; etude_dimpact: list -> tuple

• Makefile: fix run and add install and dev

• add regions.json

• st_utils: add update_submission