Print the individuals of the family in ped format
        
        The header will be the original ped header plus all headers found in
        extra info of the individuals

def to_ped(self, outfile=None):
        """
        Print the individuals of the family in ped format
        
        The header will be the original ped header plus all headers found in
        extra info of the individuals
        """
        
        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)
        ))
        
        if outfile:
            outfile.write('\t'.join(ped_header)+'\n')
        else:
            print('\t'.join(ped_header))
        
        for individual in self.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, '.'))
            
            if outfile:
                outfile.write('\t'.join(ped_info)+'\n')
            else:
                print('\t'.join(ped_info))