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!
stereo_mpi: Added a multiprocess variant of stereo

This leverages the same stereo executables as the python stereo script
... however it uses the left crop window option to divy up jobs among
multiple processes instead of threads. It then merges the results back
together using GDAL build vrt.

In the future this executable will hopefully have a 'local' mode and a
'mpiexec' mode for the super computer. This will be our answer for
processing a stereo pair across multiple computers.
DGStereo clarify notation for input DEM
disparitydebug: clarify what is being written