SGP/SIROS/E12 - downwelling pyranometer data incorrect

Because of a bad "preamp cable", the data from the downwelling pyranometer
at site E12 would generally not fall below 400 W/m^2 even at night.  We don't
know exactly when this problem begins, so beware of all downwelling
pyranometer data taken before 960422.

The "preamp cable" was replaced and the problem was fixed on 960422 at about
1630.

• Downwelling Shortwave Diffuse Hemispheric Irradiance, Ventilated Pyranometer(down_short_diffuse_hemisp)
• Down-welling unshaded pyranometer voltage(down_short_hemisp)