TWP/VCEIL/C2 - optical cross-talk compensation

The ceilometer gave frequent optical cross-talk compensation warnings, was insensitive to 
high clouds, and the tropics window was deteriorated. The instrument was replaced with a 
spare.

• Vertical visibility(vertical_visibility)
• Aerosol volume backscattering coefficient at 355 nm(backscatter)
• Second lowest cloud base height(second_cbh)
• Third cloud base height(third_cbh)
• Altitude of highest signal(alt_highest_signal)
• Window contamination(window_contamination)