Harmonization of storage

EPIC: #508

Description

goals:

• harmonize and document the different kinds of storages for json-data
  • git normal/bare repo, json+tsv, json+jsonl
  • try to get rid of bare repositories, by converting to jsonl the repositories that can't be checkouted because of the too many tsv files
• harmonize and document the way fetchers write json-data
  • create dbnomics-data-model storage writer and let fetchers use it