R.S., M.S. and A.L.B. are supported by the AFOSR grant FA9550-15-1-0077. P.D. is supported by the National Foundation for Scientific Research (FNRS) and by the Research Department of the Communaute Francaise de Belgique (Large Graph Concerted Research Action). D.W. is supported by the AFOSR, grant FA9550-15-1-0162. R.S. is also supported by the James S. McDonnell Foundation. A.L.B. is also supported by the Future and Emerging Technologies Project 317 532 'Multiplex' financed by the European Commission. We wish to thank I. Georgescu for many fruitful discussions and ideas throughout this project and J. De Nicolo for support with handling the WoS data.