SGP/SIRS/E12 - NIP failure

The short_direct_normal values are mostly missing (-9999) starting when the datastream 
came back from an extended period of no data files.  Ops determined the NIP 
(short_direct_normal datastream) instrument had failed and it was replaced.

Values of short_direct_normal can be reconstructed using the following relationship 
applied for each time period:
short_direct_normal = (down_short_hemisp - down_short_diffuse)/cos (zenith angle)

• Instantaneous direct normal shortwave irradiance, pyrheliometer thermopile
  voltage(inst_direct_normal)

• Shortwave direct normal irradiance, pyrheliometer(short_direct_normal)
• Shortwave direct normal irradiance, pyrheliometer, minima(short_direct_normal_min)
• Shortwave Direct Normal Irradiance, Pyrheliometer, Maxima(short_direct_normal_max)
• Shortwave Direct Normal Irradiance, Pyrgeometer, Standard Deviation(short_direct_normal_std)