Use smtk::model::Group API to add entities
The method smtk::model::Resource::addToGroup() does not do the same thing as smtk::model::Group::addEntity(), and the former results in a group not having access to its entities. This MR simply changes the test 'modelAttributes.py' to use the Group API, since this bug was causing test failures once another bug was addressed.
TODO: why are these two code paths not equivalent? Should model::Resource's API result in a model group having access to its entities? If so, this needs to be fixed.