Make simulations more flexible

It should be easier to switch boundary conditions and initial conditions in the surface and groundwater solver. This makes it easier to implement benchmark examples without fiddling with solver internals. Particular ToDos:

• use Dependency Injection for boundary conditions, source terms, initial conditions (perhaps more?) in groundwater & surface solvers
• coupling layer should be seen as a special case of a boundary condition (groundwater) or source term (surface)
• allow for time-dependent boundary conditions -> solvers should keep track of time themselves