Add enthalpy flux terms to CAM #522

[PeterHjortLauritzen]

Enthalpy flux terms are already being computed in the new coupler code used with MOM6. The corresponding terms need to be added in CAM.

For details see

Reconciling and improving formulations for thermodynamics and conservation principles in Earth System Models (ESMs)

https://www.overleaf.com/read/fgxczmxqfkzg