Zhao et al. 2006 for NSHMP 2008 US model

Implement Zhao et al. 2006 GMPE for subduction interface events as defined by the NSHMP 2008 US model (https://bugs.launchpad.net/oq-hazardlib/+bug/1290391)

Tests are running here: https://ci.openquake.org/job/zdevel_oq-hazardlib/45/