HFE/MET/M1 - Intermittent PWD data

The Present Weather Detector had intermittent missing values throughout this deployment.  
Missing values occurred almost everyday, usually just one or two minutes per day.

• PWD 10 minute mean visibility(pwd_mean_vis_10min)
• PWD 1 hour present weather code(pwd_pw_code_1hr)
• PWD alarm(pwd_err_code)
• PWD 1 minute mean visibility(pwd_mean_vis_1min)
• PWD 15 minute present weather code(pwd_pw_code_15min)
• PWD 1 minute mean precipitation rate(pwd_precip_rate_mean_1min)
• PWD instantaneous present weather code(pwd_pw_code_inst)
• PWD cumulative liquid precipitation(pwd_cumul_rain)