disconnected graph with measured nodes

[gabora]

unconnected measured nodes should be reported with a meaningful error message.

3.1 ap1 is separated from the network (deleted cjun 1 ap1 connection)
strange error message: "1"

3.2 ap1 is connected to node dummy , which has no incoming edge.
both nodes are recognised for compression (wrong -- ap1 is measured)
but optimisation and simulation works, but ap1 disappears and corresponding simulation is 0.

unconnected_nodes.zip