Using `iris.FUTURE.netcdf_promote` to load sea level data as cube.

@rsignell-usgs Can you review this?   You have a better eye to judge what is happening to the output than me.

http://nbviewer.ipython.org/github/ocefpaf/system-test/blob/81d7f6b268a0276f2014409477bcec5f9ac50f26/Theme_2_Extreme_Events/Scenario_2A_Coastal_Inundation/Scenario_2A_ModelDataCompare_Inundation/Scenario_2A_Water_Level_Signell.ipynb