[Sfm2] Add possibility to fix extrinsics in IncrementalSfM2 #1851

Add a new command line control to the SfM_engine to set the Extrinsic BA
parametrization to NONE or ADJUST_ALL.
- This new setting can allow to triangulate a match file from know
camera pose and refine only 3D positions of the structure within the  BA
of the SfM Engine.