clean up some const_cast'able warnings
clean up some const_cast'able warnings

mtCarto/PDAL,lucadelu/PDAL,Sciumo/PDAL,verma/PDAL,lucadelu/PDAL,Sciumo/PDAL,boundlessgeo/PDAL,verma/PDAL,Sciumo/PDAL,radiantbluetechnologies/PDAL,mpgerlek/PDAL-old,Sciumo/PDAL,DougFirErickson/PDAL,lucadelu/PDAL,verma/PDAL,radiantbluetechnologies/PDAL,radiantbluetechnologies/PDAL,boundlessgeo/PDAL,DougFirErickson/PDAL,verma/PDAL,boundlessgeo/PDAL,jwomeara/PDAL,mtCarto/PDAL,mtCarto/PDAL,boundlessgeo/PDAL,mpgerlek/PDAL-old,verma/PDAL,radiantbluetechnologies/PDAL,mpgerlek/PDAL-old,verma/PDAL,mtCarto/PDAL,DougFirErickson/PDAL,jwomeara/PDAL,jwomeara/PDAL,mpgerlek/PDAL-old,jwomeara/PDAL,DougFirErickson/PDAL,lucadelu/PDAL,verma/PDAL