Errors when outputting connection_avg_throughflow or connection_avg_intact_throughflow

[nhniina]

Reporting connection_avg_throughflow or connection_avg_intact_throughflow causes an error with unidirectional connections.

Outputs connection_avg_throughflow and connection_avg_intact_throughflow should only be written for bidirectional connections with two nodes.

There could perhaps be a warning if there are unidirectional connections or connections with more than two nodes and the user is trying to output connection_avg_throughflow or connection_avg_intact_throughflow.

[DillonJ]

I think connection_avg_throughflow could still be useful for unidirectional connections since it reports a single flow associated with the connection rather than two flows associated with the connection__node relationships. The solution instead could be to make it work for the unidirectional case also which shouldn't be too difficult.