GAN/SKYRAD/M1 - PIR2 failure

Increasing difference between PIR1 and PIR2 beginning December 12.  Suspected problem with 
PIR2 thermistor bridge or instrument thermistor.  OPS replaced PIR2 unit with a spare. 
The data from PIR2 after replacement appears to be normal.

Use PIR1 data for the specified period.

• Instantaneous Downwelling Pyrgeometer Case Thermistor Resistance, Pyrgeometer2(inst_down_long_hemisp2_case_resist)
• Instantaneous Downwelling Pyrgeometer Thermopile, Pyrgeometer2(inst_down_long_hemisp2_tp)
• Instantaneous Downwelling Pyrgeometer Dome Thermistor Resistance, Pyrgeometer2(inst_down_long_hemisp2_dome_resist)

• Downwelling Longwave Hemispheric Irradiance, Pyrgeometer2(down_long_hemisp2)
• Instantaneous Downwelling Pyrgeometer Dome Thermistor Temperature, Pyrgeometer2(inst_down_long_hemisp2_dome_temp)
• Downwelling Longwave Hemispheric Irradiance, Pyrgeometer2, Standard Deviation(down_long_hemisp2_std)
• Instantaneous Downwelling Pyrgeometer Case Thermistor Temperature, Pyrgeometer2(inst_down_long_hemisp2_case_temp)
• Instantaneous Downwelling Pyrgeometer Thermopile, Pyrgeometer2(inst_down_long_hemisp2_tp)
• Downwelling Longwave Hemispheric Irradiance, Pyrgeometer2, Maxima(down_long_hemisp2_max)
• Downwelling Longwave Hemispheric Irradiance, Pyrgeometer2, Minima(down_long_hemisp2_min)