In GraphGroup, allow loading external data

As mentioned in utopia#31 (closed), the GraphGroup should be extended to allowing to load properties into a networkx graph from data stored outside the GraphGroup.

Proposal

• Add another GraphGroup method (or extend the existing set_<node/edge>_property methods) to allow setting graph properties from containers/groups not in the GraphGroup. Perhaps integrate in the create_graph method. Open for discussion!

• Provide verbose errors and warnings, since it is the user's responsibility to take care of things like ordering of the data

How to test the implementation?

Extend GraphGroup tests, load different types of external data.