Core: Added parallel interest point matching

This is not as fast it could be as I'm by default mutexing all
interaction with the camera model. (This is because of ISIS.)
Local homography, first part
stereo.cc: User safety check for local homography
stereo_mpi: Support local homography
Local homography
Rename LocalDisparity to LocalHomography
Rename LocalDisparity to LocalHomography
Rename LocalDisparity --> LocalHomography
LocalHomography: Multi-threading, but for now turned off
StereoSessionDG: Minor formatting
Add timing statements in places