Support for 7-digit DSIDs

Draft proposal discussed in GIT meeting:

• Stick to current repo, look into sparse checkouts in case response time becomes unsatisfactory
• keep 6-digit structure as is, group 7-digits in millions, then in thousands
• Retain grouping by generator (?) for 7-digits (15xxxxx Madgraph, 16xxxxx Powheg etc.)
  • can use 10xxxxx - 14xxxxx for large BSM scans for instance
• keep using 6-digits for now, but introduce simple mechanism to disable 6-digit booking altogether at some point