support writePipeline transparently writing to stdout via special filename
support writePipeline transparently writing to stdout via special filename

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