Implement Atkinson and Boore 2006 GMPE for 2008 US model

Implement Atkinson and Boore 2006 GMPE for 2008 US model (https://bugs.launchpad.net/oq-hazardlib/+bug/1288146).

Two new classes have been added corresponding to the GMPEs for 140 and 200 bar stress drop.

Some refactoring was need to avoid code duplication.

Tests are running: https://ci.openquake.org/job/zdevel_oq-hazardlib/39/