[FEATURE] - add elevation to models for section and cutting surfaces

[lachlangrose]

Is your feature request related to a problem? Please describe. Add in a digital elevation model to the model area either:

1. interpolate a scalar field where underground < 0 and above ground >0
2. a 2d function where the z value is evaluated.

Integrate into section viewer and into the clipping of surfaces - option 1 is preferable here.