[rig] added a functor to refine only the pose of camera rig
[rig] added a functor to refine only the pose of camera rig

* each camera has a subpose which stays fix, only the pose of the rig
changes

poparteu/openMVG#13

poparteu/openMVG,poparteu/openMVG,poparteu/openMVG,poparteu/openMVG