Feature toggling

[wpbonelli]

Add a feature toggle mechanism to allow models/features to live in the develop branch before they are ready for inclusion in official releases. This involves a few capabilities:

• [x] disable dev features in release mode
  • #1384
• [ ] #1532
  • #1607
  • #1603
• [ ] filter code/docs/DFNs/examples for distribution
  • #1540