Return a generator with the info in ped format.
        
        Yields:
            An iterator with the family info in ped format

def to_ped(self):
        """
        Return a generator with the info in ped format.
        
        Yields:
            An iterator with the family info in ped format
        """
        
        ped_header = [
            '#FamilyID',
            'IndividualID',
            'PaternalID',
            'MaternalID', 
            'Sex',
            'Phenotype',
        ]
        
        extra_headers = [
            'InheritanceModel',
            'Proband',
            'Consultand',
            'Alive'
        ]
        
        for individual_id in self.individuals:
            individual = self.individuals[individual_id]
            for info in individual.extra_info:
                if info in extra_headers:
                    if info not in ped_header:
                        ped_header.append(info)
        
        self.logger.debug("Ped headers found: {0}".format(
            ', '.join(ped_header)
        ))
        
        
        
        yield '\t'.join(ped_header)
        
        for family_id in self.families:
            for individual_id in self.families[family_id].individuals:
                individual = self.families[family_id].individuals[individual_id].to_json()
                ped_info = []
                ped_info.append(individual['family_id'])
                ped_info.append(individual['id'])
                ped_info.append(individual['father'])
                ped_info.append(individual['mother'])
                ped_info.append(individual['sex'])
                ped_info.append(individual['phenotype'])
            
                if len(ped_header) > 6:
                    for header in ped_header[6:]:
                        ped_info.append(individual['extra_info'].get(header, '.'))
            
                yield '\t'.join(ped_info)