Sessions: Drop support for LUT and use Transform objects instead

THIS IS A MAJOR BREAKING CHANGE. Instead of using LUT images and
having a lot of repeated code in stereo_tri. We now instead use
transform objects provided by the sessions.
Sessions: Add TestInstantiation

Makes sure that at least the sessions compile and define the types we
need. This is to make the developers life easier so they can look at
errors here instead of errors *20 inside the Tools directory.
stereo: Remove templates in calls to stereo_triangulate

Easier to read and less to type.
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
stereo_tri: Rm stray comment
DG: Add RPC Map project test
stereo.cc: Transform the box properly
stereo: Use forward_bbox instead of just projecting 2 corners.
stereo_tri: Drop cropping of PC file.

This breaks assumptions in point2dem.
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.
RPC: Have Map Transform listed as a convex function
stereo_tri: Fix caching problem