Support 2d / 3d simulation similar to GETM

Currently, we can only simulate 1d advection / diffusion processes. This corresponds to the simulation of a vertical water column in GOTM (General Ocean Turbulence Model). GETM (General Estuarine Transport Model) is an extension of GOTM that adds horizontal advection and diffusion processes for a 2d / 3d simulation. Our model should be extended in a similar way so that it becomes possible to simulate transport processes in rivers, estuaries or oceans.