Make it possible to directly converte a rupture into XML

Before this pull request it was not possible to convert a single rupture into XML, it was possible to convert only a whole Stochastic Event Set of them. This helper is convenient in view of removing the ruptures from the database as pickled objects. We could store them as XML instead, by making use of rupture_to_element. This is convenient also for a new feature Vitor wants, i.e. the ability to run risk from a file of ruptures.