Restrict ComplexFaultSourceSurface to not pathological cases

A common user error is to specify the vertexes in the wrong order. This will trigger errors like in https://bugs.launchpad.net/openquake/+bug/1154024

A solution to this problem is to restrict the possible configuration of a complex fault source surface to only the one such that the projected enclosing polygon is a simple polygon (not multipolygon allowed).