Metadata for "Height" variable field

The metadata defining the variable "Height" is incorrect.  The metadata 
states that "Height" is the altitude of the BOTTOM of the range bins.  
In fact, "Height" is the altitude to the MIDDLE of the range bins.  
This amounts to an interpreted difference of 150 meters, since the range 
bins are 300 meters in width.  The actual data values are unchanged, only 
interpretation of the altitude is different.  This pertains to all existing 
data produces from the MPL02 at the SGP site.

• base time(base_time)
• Filter Temperature(filter_temp)
• Array of heights for the range gates(height)
• lon(lon)
• Cloud base height, measured by MPL(cloud_base_height)
• Laser Temperature(laser_temp)
• Detector Temperature(detector_temp)
• Instrument Temperature(instrument_temp)
• Number of laser shots recorded(shots_summed)
• Binary data quality flag(cloud_base_height_dq_flag)
• Repetition Rate, or Trigger Frequency of the laser(pulse_rep)
• lat(lat)
• Aerosol backscatter coefficient at 355 nm(backscatter)
• Voltage level which operates the energy monitor(voltage_15)
• Time offset of tweaks from base_time(time_offset)
• Dummy altitude for Zeb(alt)
• Voltage level which operates the thermistor(voltage_10)
• Laser output energy per pulse(energy_monitor)
• Background Signal(background_signal)
• Voltage level which operates the A/D card and the detector(voltage_05)