Implements Campbell & Bozorgnia (2008) GMPE

Implements the Campbell & Bozorgnia (2008) GMPE for Spectral Acceleration, Peak Ground Acceleration, Peak Ground Velocity, and Peak Ground Displacement. from the NGA-West 1 project. Test verification tables constructed using the Fortran code supplied by the authors, and made available on the NGA Website (http://peer.berkeley.edu/ngawest/nga_models.html). Also includes coefficients for Cumulative Absolute Velocity (CAV), taken from the Campbell & Bozorgnia (2010) paper, which adopts the same functional form

Includes:
1) Main GMPE class
2) Tests and verification tables (separate tests for reverse, strike-slip and normal faulting conditions)
3) Adds CAV to the list of supported IMTs
4) Includes PGD in the list of acceptable headers in the test suite.