Adds zed_scalefac to the output netcdf file.

[daringli]

zed_scalefac lets us calculate zeta (as defined in the stella.geometry file) from zed, allowing geometry files to be fully reconstructed from the stella netcdf output. Zeta is useful when we want to use a geometric quantity (such as gbdrift) from a different geometry file or stella simulation, as it lets us potentially interpolate to the zeta grid of the new simulation. Being able to do this construction from the output netcdf file is more precise than using the plaintext .geometry file, as the plaintext .geometry file only saves these quantities with a few digits precision.