Adding get_fault_vertexes_3d in hazardlib/geo/surface/simple_fault.py

Adding a method called get_fault_vertexes_3d  inside hazardlib/geo/surface/simple_fault.py.

The method is for calculating the coordinates (Lons., Lats., Depths )of a given fault surface.

Test are running here: https://ci.openquake.org/job/zdevel_oq-hazardlib/111/