[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.
why are you checking  `_nodes[last_layer_offset+i]` here?