Implements Campbell & Bozorgnia (2014) NGA-West 2 GMPE

This PR implements the NGA-West 2 GMPE of Campbell & Bozorgnia (2014). The GMPE contains regionalisation and is therefore broken into six classes:

i) the "global (California)" case, 
ii) Model i) modfied for the high Q (low attenuation) case (i.e. China, Turkey)
iii) Model i) modified for the low Q (high attenuation) case (i.e. Japan, Italy)
iv) Model i) modified for the case of a "Japan" basin model
v) Model iv) modfied for the high Q (low attenuation) case (i.e. China, Turkey)
vi) Model iv) modfied for the low Q (high attenuation) case (i.e. Japan, Italy)

The test tables are generated from a modified form of the implementation by Yue Hua, available here: http://web.stanford.edu/~bakerjw/GMPEs.html