Given a selection, return the vertex IDs of the vertices on the boundary of this selection. A selected vertex 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 a triangle element that is not part of the selection (i.e. if the vertex has a neighbor vertex not in selection for a selection of type EGeometryElementType::Vertex, or an adjacent edge not in selection for a selection of type EGeometryElementType::Edge, an adjacent triangle not in the selection for a selection of type EGeometryElementType::Face).