smtk::model::Group: fix flags to accept boundaries of dimension 1
In smtk/model/testing/python/modelAttributes.py:137, a group created with flag
flags = smtk.model.MODEL_BOUNDARY | smtk.model.DIMENSION_1
results in the rejection of edges being added to the group, but flag
flags = smtk.model.EDGE
succeeds.
See !1609 (merged)