prepare dedicated methods to read a LogicalGraph and GraphCollections

[galpha]

Using the CSVDataSource its possible to read a LogicalGraph from a once written GraphCollection. Right now, only one of the stored GraphHeads will be used for the final graph (remaining will be lost). We should prepare dedicated methods to read a LogicalGraph or a GraphCollection which tests the number of graphs containing in the data.

Please add possible solutions underneath (see discussion in #974).

[timo95]

We could add a info file to the graph folder containing general information about the graph when writing it (version, format, lg/gc, etc.). This would make distinguishing LogicalGraphs from GraphCollections for all new graphs trivial and further enable easy graph format transitions with the version number.