Initial implementation of Chiou & Youngs (2014) GMPE  

Implements GMPE of Chiou and Youngs (2014) for active shallow California like region. This version is the GMPE without directivity. 

Test are running here: https://ci.openquake.org/job/zdevel_oq-hazardlib/113/

Please indent by four spaces after the :returns: Also, the plural of vertex is vertices ;-)