Let logical graphs extend graph collections?

[timo95]

Looking at EPGM, logical graphs are special cases of graph collections with only one graph.

At the moment, the implementation of operators doing essentially the same to logical graphs and graph collections (CSVDataSource/Sink, Aggregation, Subgraph, etc., e.g. see #1233) results in code duplication. One common interface for getEdges, etc. that is usable in operators would solve this.

Going further, we could also let logical graphs extend graph collections and be able to apply all operators on logical graphs. I don't think, this is very practical, since some graph collection operators might be applicable to logical graphs, but not practical (Distinct, Limit, Selection).