Align time scales of ground & surface solvers

While reading through List & Radu, 2016, I stumbled across "The time unit is 1 day and dimensions are given in meters." (p. 349). We are using the same material parameters as they are, meaning that the groundwater solver actually sees dt=1 as dt=1d, whereas the SWE solver would interpret it as dt=1s.

We need to take care of this! I think it makes most sense to treat time in seconds everywhere in the code for consistency. But then I will somehow have to adapt the soil parameters/van Genuchten relations (?) to reflect the change of scales.