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:

• disable dev features in release mode
  • refactor: generalize routine to disable development features #1384
• build-time model selection unnecessary at this time
  • refactor: matching dfn and module filenames #1607
  • refactor(dfn2f90): use dfns.txt input file, add options #1603
  • feat: configure supported models at build time #1532
• filter code/docs/DFNs/examples for distribution
  • refactor(mf6ivar): discover dfns dynamically #1745
  • ci(release): support model selection for docs/dfns/examples #1746
  • ci(release): fix model selection options for dist scripts #1752