added acceptance test for stochastic event set generator
removed tom from rupture iterator in stochastic_event_set_poissonian and updated tests
renamed stochastic_event_set_poissonian to stochastic_event_set and removed time_span from method signature
removed time span also from doc
added test for stochastic event set generation with non-parametric source
method 'sample_number_of_occurrences' is now an abtract method in BaseProbabilisticRupture + added method implementation in NonParametricProbabilisticRupture
acceptance test for stochastic event set generation with non parametric source now passes
small test refactoring
removed time span from disaggregation calculator
fixed doc
Merge pull request #162 from gem/fix-calcs

Remove 'time span' from stochastic event set and disaggregation calculators