Make the documentation match reality as far as model ownership
goes (it did not show Model→Group relations and incorrectly had
submodels related by `INCLUDES/EMBEDDED_IN` instead of
`SUPERSET_OF/SUBSET_OF`).

Add methods to EntityRef for managing superset/subset arrangements
and use those (instead of raw relations) for parent-child of Model
and Session.

Rename ModelEntities and GroupEntities typedefs to Models and Groups.

Clean up Read operator in all bridges to use new session storage
style (where `Entity::relations()` holds session children) instead
of having Session instance track top-level models.

Get rid of `smtk::model::Manager::m_modelSessions`; this information
is now stored in the sessions's `Entity::relations()`.