Now ParametricProbabilisticRupture is unpickled correctly

Due to the usage of ``__slots__`` (an evil memory optimization that does a lot of bad things) ParametricProbabilisticRupture were not unpickled correctly. With this fix it is possible to implement distribution-by-rupture. The idea is to apply such strategy to ComplexFaultSources.

The tests are running here: https://ci.openquake.org/job/zdevel_oq-hazardlib/169