ValidationSubsystem

[KathleenDollard]

This is in draft because it is the entire implementation and thus could use some discussion as part of the review - perhaps in our Tuesday slot. Also, because it will have a potentially nasty rebase, although I have been rebasing onto the previous PR.

#2476 explains the approach

[KathleenDollard]

Updates

• Respond to PR review
• Add RangeBound which incorporates ValueSource to generalize 4 types of boundaries
  • ValueSource is separate so that it can be unified with default values, although that is for a future PR
  • There may be more to do to smooth out this area
• Tests for validation via values, calculations, calculations relative to other values, and calculations relative to environment variables

See ValidationSubsystemTest to see how this works.

I am out of time, and want to review myself before removing draft status.