added NZ specific McVerry et al 2006 GMPE for Asc, SInter, SSlab and Volc

Added implementation of New Zealand specific GMPE McVerry et al 2006 for TRT Active Shallow Crust, Subduction Interface, Subduction Intraslab and Volcanic. Based on the paper:

G. McVerry, J. Zhao, N.A. Abrahamson, P. Somerville published as "New Zealand Acceleration Response Spectrum Attenuation Relations for Crustal and Subduction Zone Earthquakes", Bulletin of the New Zealand Society for Earthquake Engineering, v.39, no. 1, p. 1-58, March 2006.

URL: http://www.nzsee.org.nz/db/Bulletin/Archive/39(1)0001.pdf
Last accessed 10 September 2014.
I added a new method _compute_mean which bundles this logic together and is now called 3 times in _get_mean_and_stddevs