Youngs et al. 1997 for NSHMP 2008 US model

Implement Youngs et al. 1997 for subduction interface events for 2008 US hazard model (https://bugs.launchpad.net/oq-hazardlib/+bug/1290800)

Tests are running here: https://ci.openquake.org/job/zdevel_oq-hazardlib/46/