documentation change for pressure variable

The XDC documentation for the pressure variable is incomplete- it reads
"pressure" rather than "pressure reduced to sea-level".  It has always
been sea-level pressure although the provider of the data to the XDC (NCDC)
documents it incorrectly as surface pressure.

Change the XDC documentation and netcdf header information for the pressure
longname to read "pressure reduced to sea-level".

• north beam elevation angle(ebeam_el_stn)
• site number(site_num_stn)
• vertical beam elevation angle(vbeam_el_stn)
• Wind Speed(wspd)
• Relative Humidity (hybrid-b grid)(rh)
• north beam azimuth angle(nbeam_az_stn)
• Environmental Temperature (hybrid-b grid)(temp)
• Pressure reduced to mean sea level(press)
• Station altitude(alt_stn)
• Anemometer indicator(anem_ind)
• -180.0 - +180.0(alt)
• Time offset from base_time(time_offset)
• orientation from true N(orient_n_stn)
• surface rainfall count(rain)
• Profiler in checkout mode(checkout_mode)
• north beam elevation angle(nbeam_el_stn)
• east beam azimuth angle(ebeam_az_stn)
• lon(lon)
• Dewpoint Temperature(dp)
• vertical beam azimuth angle(vbeam_az_stn)
• Wind Direction(wdir)
• Count of 6-minute periods to compute average(avg_count)
• North Latitude(lat)
• Station longitude(lon_stn)
• base time in epoch(base_time)
• Station Latitude(lat_stn)
• magnetic declination(magnetic_dcn_stn)