disconnectFromGraph

If a node should be removed from the graph, just removing it from the AST is not enough (see issue #60). It will most probably be referenced somewhere via DFG or EOG edges. Thus, if it needs to be disconnected completely, we will have to take care of correctly disconnecting these implicit edges.

ATTENTION! Please note that this might kill an entire subgraph, if the node to disconnect has further children that have no alternative connection paths to the rest of the graph.