DO NOT MERGE: The enthalpy form of the heat equation

[martynpclark]

Make sure all the relevant boxes are checked (and only check the box if you actually completed the step):

• [ ] closes #xxx (identify the issue associated with this PR)
• [ ] tests passed
• [ ] new tests added
• [ ] science test figures
• [ ] checked that the new code conforms to the SUMMA coding conventions
• [ ] ReleaseNotes entry

[martynpclark]

The majority of this PR is to incorporate enthalpy as a diagnostic variable. It also implements the enthalpy form of the heat equation by using the finite difference estimate of volumetric heat capacity.

TODO:

1. Add a switch to use the temperature form of the heat equation;
2. Show results from science testing.

[martynpclark]

[martynpclark]

Part of the SUNDIALS merge