Added a new parameter complex_fault_mesh_spacing

By using this parameter, it is possible to use a larger mesh spacing for ComplexFaultSources, thus reducing the  number of generated ruptures and the computation time. For instance, for our qa_test/classical/case_5, by raising complex_fault_mesh_spacing from 0.01 to 0.05 the total runtime goes down from 421 seconds to only 6 seconds.

The tests are green: https://ci.openquake.org/job/zdevel_oq-risklib/109/