Decisions on filter output conventions

[cchwala]

Some decision regarding convection of the output of the filter functions, based on discussion with @lepetersson and @JochenSeidel.

Meaning of flag values:

• filter output is 1 for detecting an anomaly
• filter output for functions stemming from the PWSQC R code is -1 in case the filter cannot be applied because not enough (non-NaN) data from neighbors
• filter output is 0 otherwise

Data handling:

• we write the flag time series to the xarray.Dataset as new variable
• raw data is not changed
• this way the user can decided, after generating/computing the flags, if all flags should be applied and how