geomatrix 1993 GMPE for subduction intraslab

Implement geomatrix 1993 GMPE for subduction intraslab models used by both the 2008 US and 2010 South America models (https://bugs.launchpad.net/oq-hazardlib/+bug/1289337)

Tests running here: https://ci.openquake.org/job/zdevel_oq-hazardlib/43/