Improved treatment of mixed-phase precipitation

Doing mixed-phase treatment when 0 < PSUM_PH < 1: in this case, the liquid water will be distributed over the new snow layers. Not more than 50% of pore space in the new snow can be filled by water.

A new key ALLOW_FRREZING_RAIN is introduced. If set to TRUE, precipitation is assumed to be at air temperature or melting point, whichever is lower. The temperature of new snow elements during mixed precipitation is set accordingly. If set to FALSE, precipitation is assumed to be at melting point, and new snow elements during mixed precipitation are also initialized at melting point, even when air temperature is below freezing.