xyz_to_cid optional z

[tomvansteijn]

Currently there are two functions mapping x,y-coordinates to model coordinates: xyz_to_cid mapping x, y and z to layer, row, column on a structured grid or icell2d on a vertex grid, and xy_to_icell2d mapping x and y to icell2d on a vertex grid. I think it would be easier if to have one function with an optional z argument, allowing the user to enter the layer separately.