SGP/SIRS/E7 - Downwelling Shortwave failure

The downwelling shortwave (Global PSP) values were dropping out.  Problem started 2/4 but 
became persistent 3/9 and continued until replacement of Mux on 5/15.

PSP data (down_short_hemisp) is not recoverable but can be calculated using:
global = direct*cos(zenith angle) + diffuse 

See the SIRS instrument handbook for details of this component sum method.

• 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)