point2dem: Improve support for different sources of datums
point2dem: Improve support for different sources of datums

Prior to this change the user could have done:

point2dem -r moon -t_srs "+proj=eqc lat_0=26"

This would have been goofy because the proj4 string being parsed by
point2dem would have both the axis of the moon plus the default datum,
WGS 84, from GDAL being applied. It wasn't exactly clear what values
were be used to process the data.

This fix seems to correct that, while still allowing the user to use
all the complex features provided to them by Proj4.

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