unit of variable ssf

[verseve]

The variable lateral subsurface flow ssf has unit m3 Δt-1 (model timestep), while other flow variables have unit m3 s-1. The ssf state is now not independent of the model timestep and for example the ssf state of a daily run needs to be changed manually to use it for an hourly run. The unit of ssf should be changed in the code.