AC and RA acknowledge support from the U.K. Science and Technology Facilities Council (grant number ST/N000927/1) as does AL (grant number ST/L000652/1). AL and JC acknowledge support from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement No. . J.G.N. acknowledges financial support from the Spanish MINECO for a ‘Ramon y Cajal’ Fellowship (RYC-2013-13256) and the I+D 2015 project AYA2015-65887-P (MINECO/FEDER).” CJM is supported by an FCT Research Professorship, contract reference IF/00064/2012, funded by FCT/MCTES (Portugal) and POPH/FSE (EC). Some of the results in this paper have been derived using the HEALPix package.