Ab2003 nshmp

Implement AB2003 GMPEs for US and South America USGS-NSHMP hazard models (see https://bugs.launchpad.net/oq-hazardlib/+bug/1289216)

Tests are running: https://ci.openquake.org/job/zdevel_oq-hazardlib/42/

Small refactoring of existing method was needed to avoid code duplication