Ensure a reliable way to distinguish surface items with elevation values (2,5D grids) and property values (color mapping)

Suggestion:
- Enforce that the `content` is set and set correctly for all surface items. Do not allow empty `content`.
- Only surfaces that has elevation values should be tagged `depth`
- Surface with content `depth` that is mapped to a official stratigraphic unit must link to that units `top` or `base` name. It must not be linked to the `stratigraphic unit identifier` as that represents a volume.