Relabel coordinates so that the buoyancy forcing varies in `y` rather than `x`

[hdrake]

This would make the orientation of the coordinates more consistent with both the meridional overturning circulation in the ocean (where y ~ latitude) and the rest of the Horizontal Convection literature

[hdrake]

If you're going to do this, probably best to do it first since it's a pretty major change to all of the scripts and notebooks, etc.!