Now PointsBuilder's terrainExtraction ability is an operator in ModelBuilder.
It would hide the input aux_geom and load the result into ModelBuilder.

This MR introduces new SMTK optional dependencies as VXL and rtvl which
terrain extraction operator depends on. Some CMB classes are copied into
SMTK(vtkLIDARElevationFilter, rtvl library, vtkTexturePointIntensityFilter,
vtkLIDARPtsWriter, vtkHydroModelPolySource and
vtkTerrainEtractionFilter.) CMB should delete these duplicated classes for
simplicity.

One thing worth mentioning is that minimum spacing computation is a
simplified version of it's PointsBuilder counterpart. ClipPolygons and
PointThresholdFilter is not used.