provide the option for transport to take sources as evaluators

[ecoon]

Transport relies on capabilities around Amanzi's DomainFunctions to provide non-linear source terms. Domain Functions could have been evaluators, but are not.

This has both advantages (lower memory footprint) and disadvantages (no ability to visualize or observe individual components of the source terms; style mismatch with the rest of ATS).

This PR implements SOME of the domian functions of Amanzi as evaluators. It then swaps to using those in transport. Now, transport source terms may be provided in either or both ways:

• using the PK's "source terms" sublist --> DomainFunctions
• using the PK's "has source term", "source term key" to provide sources as an evaluator.

Geochemistry still must be provided as a DomainFunction. Standard sources independent of mole fraction may be provided either way.

The ATS input converter changes some of these to evaluators.