[BBPBGLIB-1079] Ability to override connection parameters of a specific edge population

[WeinaJi]

https://bbpteam.epfl.ch/project/issues/browse/BBPBGLIB-1079

Connection override blocks allow overriding certain properties of edges.
The selection of the edges can be done by selecting the source and destination nodesets.
This ticket if to allow more fine grained selection of the edges. Namely, after BBPBGLIB-1061 where several edge populations can be loaded, it is desirable that one can specify the overrides on only one of these edge populations.
Q. What should the they keyword to select the edge population? Maybe "edges"? E.g.
...{
"override-single-edges": {
"source": ...,
"edges": "edge_population_A",
"synapse_configure": ...
}