return const& for getField requests instead of copies for performance win
return const& for getField requests instead of copies for performance win

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