Conversion of LayerFracSolidPrecip in SeriesAggreg

LayerFracSolidPrecip is converted through a sum in SeriesAggreg. LayerFracSolidPrecip is a fraction, so it cannot be converted with a sum. An average would be less incorrect, although definitely not perfectly correct. A better way would be to recalculate liquid and solid precipitation, to aggregate them, and then to recalculate LayerFracSolidPrecip as the ratio.