Improved connector performance

There was a tiny bug in the _target_pos_and_vec_defined method which has been fixed. This has significantly improved performance for this method, so the accuracy can be improved (if required) by modifying the Connector class property delta_radius to e.g. 1.0 (from the default value of 20.0).

This should also solve any recursive limit issues since solutions are now being found within << 100 iterations (on average less than 10).