WIP: The units of the main governing eq are inconsistent - see EP1950

Applies sources on temperature ('T' - incompressible)
or energy ('h'/'e' - compressible) equation to incorporate
evapotranspiration heat-transfer effects from the specified
plant canopy. Heat transfer is usually calculated based on
empirical relations between plants and solar radiation.

Two submodels to incorporate heat transfer effects
- 'tree': specified tree canopy - uses empirical relations between solar
radiation and evapotranspiration.
- 'grass': specified grass canopy - uses Pemnan-Monteith Equation model.