added NZ specific McVerry et al 2006 GMPE for Asc, SInter, SSlab and Volc
modified McVerry2006 GMPE as suggested by monellid
added McVerry2006 GMPE, minor revisions to pep8 styling
added McVerry2006 to index.rst
Merge pull request #232 from nickhorspool/master

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