Cannot set state variables associated with SPARSE connectivity

[neworderofjamie]

SynapseGroup.synapse_order never initialised - perhaps pulling connectivity should do this?

Additionally, specifying an array as the initialiser for a weight update model / custom update / custom connectivity update synaptic state variable should give an error if the underlying synapse group's connectivity is configured with a sparse connectivity initialisation snippet.

[gitsofaryan]

@neworderofjamie please assign me this work!!