Preparatory changes for adding water processes

This PR includes several changes and improvements which are necessary(-ish) for properly implementing water flow.

The most important changes are:

1. The addition of the timestep(::Layer, ::Process, state) method for modular definition of timestep constraints, similar to what is done in the community model.
2. The addition of the state variable domain constraints, e.g. Prognostic(:sat, OnGrid(Cells), domain=0..1). These are automatically enforced via the CryoGrid.domain function which is used as the isoutofdomain function for the diffeq solver.
3. Moving SWRC to the Soils module (it kind of belongs there anyway).