stereo_tri: Remove more unnecessary code
stereo: Have left crop win be defined by the L.tif, not left img.

The new design allows the alignment options to change the size of L
and R tifs.
Sessions: Add NadirPinhole session

This a session specifically for satellites that are modelled by
pinhole camera models but look straight down at earth from an orbit.
DG: Add back support for RPC Map Projected images

Now we provide a TransformRef that contains a composite of
RPCMapTransform and HomographyTransform.
Core: Have homography alignment crop to active areas of image

To help signify to all the developers that this is the new operation
... I changed homography_fit to homography_rectification. It kinda
meets the naming pattern I set in affine_epipolar_rectification.
Modify code and doc to reflect left and right transforms
DemDisparity: Make it work for new hom and affine epipolar changes
stereo.cc: Correct handling of left-image-crop-win
DG: Add RPC Map project test
stereo.cc: Transform the box properly
LocalHomography: Bug fix, left and right were swapped
session: Add DG Map RPC Session

This is a session specifically made so that we don't use a
TransformRef. This allows multiple copies of RPCMapTransform to be
created for each thread (w/ TransformRef a single copy would be hidden
behind a shared ptr). These multiple copies thus allow RPCMapTransform
to cache its DEM usage ... and thus Triangulation happens faster.
stereo_tri: Fix caching problem
stereo: Update automatic session guessing

Now it knows about DG Map RPC.
DemDisparity: Read align matrix only if available
StereoSessionPinhole: Bug fixes
StereoSessionPinhole: Consistency
Pinhole: Move determine image align into Pinhole's cc files

It is only used by that session. There is no need for it to be inside
the base class of stereo sessions anymore.