should the sub-dag be valid as soon as an evaluator is required?

[ecoon]

There are some important usages of the sub-dag rooted at an evaluator as soon as that evaluator is required. For instance, in a PK, one might wish to do:

S->RequireEvaluator("my source");
if (S->GetEvaluator("my source")->IsDifferentiableWRT("my primary")) {
  S->RequireDerivative<>("my source", "my primary");
}

to determine if we need derivatives in the preconditioner.

This would be accomplished by calling Evaluator::EnsureEvaluators(); immediately after construction of the evaluator via Require in State.

The downside of this is that we no longer then have options to alter the evaluator list. For instance, if a PK that goes before Richards requires a field that depends on saturation, this will currently fail because the saturation WRM info gets moved into the state evaluators list only in the Richards PK.

[ecoon]

Possible fixes -- require that all changes to the "state"->"evaluators" list to be made during PK construction, not during Setup?

[ecoon]

Note that once this is fixed, can remove the "source term is differentiable" option in conservation equation PKs.