Given a EGeometryTopologyType::Polygroup selection, return the corner IDs of the polygroup corners on the boundary of the selection. A selected corner is considered to be on the boundary either if it is on the actual mesh boundary (for an open mesh) or it is connected to an element that is not part of the selection (i.e. if there is a neighbor corner not in the selection for a selection of type EGeometryElementType::Vertex, or an adjoining edge not in the selection for a selection of type EGeometryElementType::Edge, or an adjoining group not in the selection for a selection of type EGeometryElementType::Face).