Construction of spaces incorporating topography

After some discussion with @Zhengyu-Huang and Paul, my general thought is to handle this as follows

1. Construct the horizontal space (i.e. a sphere or rectangle)
2. Construct the hybrid product space with a pseudo-vertical coordinate: this can either be η in (0,1), or it could be a reference z* in (0, `z_top`).
3. Construct a field `z_surface` on the horizontal space.
4. Construct a field `z_levels` on the product face space (this would be as a function of 3 and the pseudo-verical coordinate in 2.)
5. Take the horizontal covariant gradient of `z_levels` to get the metric terms.
6. Update the `LocalGeometry` terms to get the new space

At a user level, initially I think the process would be
1. Construct the horizontal space
2. Construct `z_surface` (either as a function, or loading from a data file: figuring out how to do this will be a separate issue)
3. Some sort of function that takes (`z_surface`, the vertical stretching, and the terrain adaptation), and returns the hybrid space