Somerville et al. 2009 GMPE for Australia

Implements Somerville et al. 2009 GMPE, needed for the implementation of the Australia Seismic Hazard Model (see https://bugs.launchpad.net/oq-hazardlib/+bug/1214762)

Two class have been defined, one for non-cratonic australia and one for the Yilgarn Craton. The latter extends the former because they share the same functional form, only the coefficients differ.

Test data for the Yilgarn Craton GMPE have been provided by Duncan Gray from Geoscience Australia. With this test data the test coverage is already 100%. As soon as I receive test data for the non-cratonic GMPE I will do a second pull request with an additional test.