Handling of NaNs in rainfall data

[lepetersson]

FZ-filter: Current version sets the flag to 1 if the last nint timesteps are NaN, PWSQC set it to zero. Example: station ams96 has only data for one month July/August 2017. It is offline the rest of the time. Pypwsqc sets the "NaN-periods" to 1, including when the station is offline for a short while in July. PWSQC code first sets the flags to -1, and once the station has been online once, it sets the flag to 1.

HI-filter: Current version sets the flag to 0 if the station is reporting NaN-values. PWSQC sets the flag to -1 when it has never been online, and 0 when it has been online and then goes offline (see example Ams96)