Mixed phase treatment

When mixed-phase occurs (PSUM_PH between 0 and 1), it could be a good approach to equally distribute the liquid water over the new snow layer. Currently, first snow is added, and later in the time step, liquid water is added. This can lead to many thin ice layers mixed in with the new snow.

For the BUCKET approach, one could simply fill up to field capacity, or the total rain amount, whatever is smaller. For RICHARDSEQUATION, this could be a bit more complex to make it work.