GETM as slice model

By chosing the compiler option SLICE_MODEL it is possible to operate GETM as a two-dimensional vertical ($xz$-)model under the assumption that all gradients in $y$-direction vanish. In order to do so, a bathymetry file with a width of 4 grid points has to be generated, with the outer ($j=1$, $j=4$) bathymetry values set to land, and the two inner ones being independent on $j$. The compiler option SLICE_MODEL then sets the transports, velocities, and sea surface elevations such that they are independent of $y$, i.e. they are forced to be identical for the same $j$-index. Especially, the $V$-transports and velocities in the walls ($j=1$, $j=3$) are set to the calculated value at index $j=2$.