Adds constraint to the ProbModelXML.

def _add_constraint(self, constraint):
        """
        Adds constraint to the ProbModelXML.
        """
        constraint_data = self.data['probnet']['AdditionalConstraints'][constraint]
        constraint_element = etree.SubElement(
            self.additional_constraints, 'Constraint', attrib={'name': constraint})
        for argument in sorted(constraint_data):
            name = argument
            value = constraint_data[name]
            etree.SubElement(constraint_element, 'Argument', attrib={'name': name, 'value': value})