Feature complete : mergeback 2d_coords

[pp-mo]

Master ticket for completing current work on 2d_coords (part of ORCA data support effort).

Rationale :

• in the end, the 2d_coords branch will eventually be ok to merge back
• in development, we will want to merge in PRs that don't meet our usual standards
  • incomplete implementation
  • known problems
  • temporary limitations
  • documentation needs fixing
  • temporary implementation awaiting other outstanding work for a better solution
• in these cases, all the outstanding work needs to be recorded in issues marked with the '2d_coords' label.
• broken tests can skipped during development using "@skip_2d" Provide a test skipper for 2d coords WIP. #3099
• once all open issues+prs labelled 2d_coords are closed (in theory),
  everything is up to standard + we can merge back.
  • but note especially : 2d_coords whatsnew #3101, Remove "skip_2d" #3102
  • in practice, it will still want a bit of an overall review

[DPeterK]

Done in #3153.