Implements Boore, Stewart, Seyhan and Atkinson (2014) NGA-West 2 GMPE

This PR implements the latest NGA-West 2 GMPE of Boore et al. (2014).

The GMPE makes heavy use of regionalisation, requiring different coefficients to describe the path scaling under "average", low Q and high Q conditions, as well as different basin response scaling terms for null conditions, a "California" scaling model and a "Japan" scaling model. Furthermore the GMPE also allows for the possibility of defining an "unspecfied" style-of-faulting, which we support with the use of sub-classes in which the style-of-faulting (rake) parameter is not required. To accommodate all of the possible regionalisation combinations, in addition to the option of the unspecified style-of-faulting, a total of 18 classes are used. 
You're using a slight different notation that the one generally adopted. See for example https://github.com/gem/oq-hazardlib/blob/master/openquake/hazardlib/gsim/zhao_2006.py#L32