SGP/SIRS/E16 - Downwelling global SW incorrect

The down_short_hemisp (PSP instrument) values exhibit a large offset due to ventilator fan 
failure and induced instrument heating.  This offset cannot be corrected.  

Users can derive an estimate of global shortwave values with the following formula:
global = direct * cos(zenith angle) + diffuse

• Instantaneous Downwelling Hemispheric Shortwave, Pyranometer Thermopile Voltage(inst_global)

• Down-welling unshaded pyranometer voltage(down_short_hemisp)
• Downwelling Shortwave Hemispheric Irradiance, Pyranometer, Minima(down_short_hemisp_min)
• Downwelling Shortwave Hemispheric Irradiance, Ventilated Pyrgeometer, Standard
  Deviation(down_short_hemisp_std)
• Downwelling shortwave hemispheric irradiance, pyranometer, maxima(down_short_hemisp_max)