Two Bundle Adjustments in Process Graph

The Bundle Adjustment class is used twice in the pipeline, which forced us to define a dummy class for two-view BA and put the global BA UiMetadata in the actual class below. See #540.

Also, small related nit, BA returns GTSFMData objects, which again mean different things in these two different contexts.

Start by locating the Bundle Adjustment class and the two places where it is used in the process graph, then read the discussion in #540. Compare the UiMetadata placement and the meanings of the returned GTSFMData objects in the two-view and global BA contexts. Done means the two contexts no longer require a dummy class and their metadata and return semantics are distinct and clear.

Written by the indexing model from the issue text.