Move to computations using a `BuoyancyField`

[jbisits]

This would remove the need for the kernel functions I have written but may be the path of least resistance given that my kernel functions are not as yet running on GPUs.

This can be done by setting a field that is the buoyancy perturbation that is calculated using the density anomaly. Alternatively I could just calculate the density using the density anomaly and adding the reference density (though this may not be necessary.

To calculate the inferred vertical mixing I would still need to calculated the buoyancy that has been interpolated onto (Center, Center, Face).