StereoSessionDG: Must check if georef is valid

- Cannot do stereo with map-projected imagery without valid georef
RPC: Allow reading of XML files from Pleiades and Astrium

They have slightly different XML files with different tags. They also
have a neat inverse model.
DG: RPC: Allow support of image paths that are in other dirs

ASP was freaking out about writing match files when the input files
were in their own directory.
NEWS: Got the year wrong

Thanks Ralph!