Most errors flagged up by the tests have been dealt with now, there are still
a couple left - one case of infinite recursion in beam tracing (again likely
to be related to the model network now storing node names as keys) and two
cases in the ABCD testing where NodeExceptions are not thrown when they should be.
This last one is due to the removal of the is_neighbour check in Connector.coupling_type
method - removed because this check causes the coupling_type attribute of network edges to
be None when first adding an edge to the graph.