Possible modes for a Selection.
mixed: any combination of vertices and edges is supported
isolated: either a set of vertices, or a set of edges, but not both at the same time
nodesOnly: only Node objects
groupsOnly: only Group objects
edgesOnly:only Edge object.
Possible modes for a Selection. mixed: any combination of vertices and edges is supported isolated: either a set of vertices, or a set of edges, but not both at the same time nodesOnly: only Node objects groupsOnly: only Group objects edgesOnly:only Edge object.