[Fix] Subsurface DEM Conflict

This resolves an issue where the DEM was considered as the most reliable part of information in a subsurface mesh building process which could result in mesh nodes located below the actual bottom layer of the model. This fix changes this in a way that all mesh nodes must now be located between top (DEM) and bottom layer.
minor comment (up to you): i think the same condition is also used somewhere above. as it is bit lengthy and not easy to get the meaning at glance, i suggest to introduce a function with a readable name like `if (isElevationUnspecified(nodes[i]))`?