Brittle smoothing parameter calculation
The calculation of smoothing parameters with the linear method is somewhat brittle: If due to repeated values (for example many zeros in precipitation) the effective number of points available for the smoothing parameter calculation is very small, the selection of only the middle 90% of points can be too small to allow for a proper linear fit, in fact, it can even be zero.
This should be taken into account in the _smoothing_parameters_linear
method.