Implement preflighting for stochastic weight updates

[sebjameswml]

E.g.:

<LL:WeightUpdate name="First Lot to Second Lot Synapse 0 weight_update" url="Weight.xml" input_src_port="a" input_dst_port="in">
  <Property name="w" dimension="?">
    <UniformDistribution minimum="0" maximum="1" seed="123"/>
  </Property>
</LL:WeightUpdate>

[sebjameswml]

I think these are currently expanded by SpineML_2_BRAHMS, but would be better expanded here in SpineML_PreFlight.