Provide fast way to determine in-degree of node

[heplesser]

Currently, determining the in-degree of a node in general requires expensive GetConnections operations. On the other hand, Archiving_Node already keeps track of the number of incoming plastic synapses. Could we extend this mechanism so that it becomes cheap to determine the in-degree of a node?�

The background for this is a request for noise proportional to the in-degree of a neuron by Adam Haber, see discussion on NEST User 23-27 Oct 2016.

[Silmathoron]

I think we can reference @alexeyshusharin's proposal (#523) here, since it directly addresses this problem, among other things.

[github-actions]

Issue automatically marked stale!