Hydrology - spinup soil moisture and accumulated flows

[vgro]

The hydrology model needs a spin up to get soil moisture and accumulated flows to a steady state before starting the simulation. For the soil moisture, this should be at least two years. For the surface flows, this should go over many more iterations, I expect 1000 steps to reach a steady state.

It is not clear at the moment which models need to run with the hydrology model spin up to make this work properly. Potentially the plants, or we can make an assumption about the plant structure and evapotranspiration. The function would either set a fixed number of steps or run until the change falls below a threshold.

The location for this process could be spinup(). I am not sure how this is meant to be working, but this would be the place to run it.
Alternatively, we could make this part of init.