Add ```z_connected``` to values to be set for synaptic elements

[ackurth]

The z_connected variables in synaptic elements in used to keep track of the connected synaptic elements, in contrast to the z variable that is used to determine how many connections ought to be established or deleted. There are use cases when one wants to set z_connected by hand (for example when creating connections with a higher level connections routine that subsequently should be subject to potential removal by structural plasticity). This PR add z_connected to the parameters that can be set by the user.

[clinssen]

@ackurth: why could this not be done through calling SynapticElement::connect()?

[github-actions[bot]]

Pull request automatically marked stale!

[heplesser]

@ackurth Could you respond to the comments above?