add comment to explain reference for ground flux equation
*
* Calculate snow pack energy balance
*
+ * Based on the SnowPackEnergyBalance function in DHSVM
+ * Reference: Bras, R.A., Hydrology, an introduction to hydrologic science,
+ * Addison Wesley, Inc., Reading, etc., 1990.
+ *
* @section LICENSE
*
* The Variable Infiltration Capacity (VIC) macroscale hydrological model
