stereo_mpi: Integrate mpiexec in to the process

This variant can correctly run a single stereo pair across multiple
machines.
RPC: Correct the XML loading for Astrium images

We want the inverse model that takes geodetic to image not their
'direct_model'.
stereo: Added stereo_corr_sub

This separates out D_sub calculation so that it can optionally happen
in stereo_corr_sub. This is important for MPI calculation so that
D_sub is not generated many processes over. This might also be the
point in the code where we make D_sub from DEM generation possible.
stereo_mpi: Use poll instead of wait

Not all stereo_corr jobs are the same. We need to poll around to see
what process we can pop. Life would be much easier if we could use
python multiprocessing ... but it is not available in 2.4.
asp: Make compliant with changes to IP matcher in VW

We dropped the argument that describes if we are bidirectional or
not. It now happens in the matcher's constructor. (Where it probably
should have always been).
IsisIO: Remove Isis headers from IsisCamera header

This should allow us to build against Isis camera models without
having to reference Isis Headers and all their ugly header
dependencies. You still need to link Isis and Isis dependencies. Just
not the headers!
StereoSettings: minor cleanup