SynapseDynamicsStructural can the intial_delay ever be a Tuple

[Christian-B]

AbstractSynapseDynamicsStructural initial_delay property was defined to return a float or tuple of float.
initial_delay
Yet SynapseDynamicsStructuralSTDP.get_delay_maximum does a max of this values and a float.

IF the intitial delay is supposed to be a min max value this and other code will need fixing.