[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.
This is testing if the elevation of the current node is larger then the node that the same location in the layer below (specifically, if the current layer has the minimum thickness required for TetGen to be able to dead with this).