[FEATURE] depth to top of unit

[lachlangrose]

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Request from @markdlindsay and @markjessell Is your feature request related to a problem? Please describe. Calculate the depth to the top of a unit

• May need to improve how dtms are integrated and be able to return a surface or grid with the z values being current height.

Describe the solution you'd like Should be as simple as evaluating the scalar field on the dtm points and determine the difference to the value of the stratigraphic unit in the stratigraphic column.