Adds population to PopulationSet

def add_population(self,pop):
        """Adds population to PopulationSet
        """
        if pop.model in self.modelnames:
            raise ValueError('%s model already in PopulationSet.' % pop.model)
        self.modelnames.append(pop.model)
        self.shortmodelnames.append(pop.modelshort)
        self.poplist.append(pop)