Added feature `stdev_cut` to the penalized spline method. The new parameter takes a floating point value. Outliers larger than `stdev_cut` are removed in each iteration. The threshold is calculated in standard deviations from the mean (after trend fitting). Older versions of wotan had this parameter hardcoded as `PSPLINES_STDEV_CUT = 2 `. Now it is user variable. If no value is given, it will fall back on the default.