11_datasets.ipynb: Add datasets from O'Reach paper

The paper in question, namely Kathrin Hanauer, Christian Schulz,
Jonathan Trummer "Faster Reachability in Static Graphs" (2020)
https://arxiv.org/abs/2008.10932 uses the same set of graph datasets
(graph instances) as the PReaCH paper, though with expanded synthetic
graph sections (which were left off and not added to the notebook).

It does include rather more detail about graph statistics.  While at it,
add information about the source of real graphs in this paper.

Author: jnareb