Correctly interpolate diurnal soil temperature range

[dschlaep]

• the translation of values of the regularly spaced soil temperature profile to the soil layer profile would be producing negative values for the diurnal soil temperature range, lyrTemp_to_lyrSoil_temperature()
• this was because the interpolation uses a simple 2-point linear interpolation whereas the the diurnal soil temperature range is dampened exponentially with soil depth

-> currently a quick patch avoids negative values; however, a better solution would be to use a more careful interpolation (probably also with a more direct approach than the weighing-scheme utilized for mean soil temperature by lyrTemp_to_lyrSoil_temperature())