[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.
is it right you assume raster data for top layer is always stored at the last of `rasters`?