point2dem: Use nodata value when writing DRGs.
point2dem: Use nodata value when writing DRGs.

Then will make things compositable with gdalbuildvrt. I also added an
option to not write out the DEM. This is for people like me who are
just constantly make orthoimages.

ScottMcMichael/StereoPipeline,aaronknister/StereoPipeline,ScottMcMichael/StereoPipeline,DougFirErickson/StereoPipeline,oleg-alexandrov/StereoPipeline,ScottMcMichael/StereoPipeline,DougFirErickson/StereoPipeline,oleg-alexandrov/StereoPipeline,DougFirErickson/StereoPipeline,NeoGeographyToolkit/StereoPipeline,NeoGeographyToolkit/StereoPipeline,oleg-alexandrov/StereoPipeline,aaronknister/StereoPipeline,ScottMcMichael/StereoPipeline,DougFirErickson/StereoPipeline,DougFirErickson/StereoPipeline,aaronknister/StereoPipeline,ScottMcMichael/StereoPipeline,njwilson23/StereoPipeline,njwilson23/StereoPipeline,njwilson23/StereoPipeline,DougFirErickson/StereoPipeline,NeoGeographyToolkit/StereoPipeline,oleg-alexandrov/StereoPipeline,NeoGeographyToolkit/StereoPipeline,njwilson23/StereoPipeline,njwilson23/StereoPipeline,oleg-alexandrov/StereoPipeline,NeoGeographyToolkit/StereoPipeline,NeoGeographyToolkit/StereoPipeline,oleg-alexandrov/StereoPipeline,njwilson23/StereoPipeline,aaronknister/StereoPipeline,aaronknister/StereoPipeline,ScottMcMichael/StereoPipeline