Collapse OSCAL 1.0.0 classes to remove __root__

[fsuits]

Currently there are 27 uses of root in all the normalized oscal models (not including target) and there are only a few different patterns of usage - mostly involving regex.

Meanwhile the trestle code and tests have many references to root, which goes against the natural flow to access variables directly by name.

The situation could be simplified by collapsing the root variables into the classes that reference them, with the downside that the description and title would be lost.