Improve calculation of geopotential at single precision

[Thomas Bendall (tommbendall)]

The geopotential field is calculated in our model (when using shallow gravity) as g*z, where z is the height above the Earth's radius R.

We have been calculating z as the radial coordinate r and then subtracting R. At single precision, this method for calculating z can be very imprecise, and can create further errors if we ever need to calculate the difference between the geopotential at two different heights.

This PR changes the calculation of the geopotential to use the precomputed height_w3 field rather than the chi fields, which should not suffer from these problems at single precision when using spherical coordinate systems. The same change is made to the "deep" gravity option, and allows the planar and spherical geometry branches of code to be combined.

This calculation should now also be more efficient because it avoids accessing the chi arrays and performing a coordinate conversion.

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