Handling upper/lower tails and extrapolation

The performance of the bias-adjustment for the lower- and uppermost percentile could be improved as there is no clear effect of the bias-adjustment visible in tests. This could be related to the way the bias-adjustment is extrapolated beyond the value range given by the data in the reference period. A possible solution discussed was to base the adjustment on the lowest/highst n-% percentile range instead of the lowest/highest values.