Account for topography in surface height

[kmdeck]

When we compute surface fluxes, we need access to z_atmos - z_land, where z_land = elevation + z_0 + displacement_height, and z_atmos = height of first interior atmos point (including elevation!). Then the difference between them is height of atmos relative to the ground + z_0 + displacement height.

Right now land does not include topography, and so it returns elevation = 0. This would need to be changed for coupled runs, but is appropriate for prescribed atmos runs.

[kmdeck]

@juliasloan25