Reqest reading of flexifunction data

                target_system             : System ID (uint8_t)
                target_component          : Component ID (uint8_t)
                read_req_type             : Type of flexifunction data requested (int16_t)
                data_index                : index into data where needed (int16_t)

def flexifunction_read_req_encode(self, target_system, target_component, read_req_type, data_index):
                '''
                Reqest reading of flexifunction data

                target_system             : System ID (uint8_t)
                target_component          : Component ID (uint8_t)
                read_req_type             : Type of flexifunction data requested (int16_t)
                data_index                : index into data where needed (int16_t)

                '''
                return MAVLink_flexifunction_read_req_message(target_system, target_component, read_req_type, data_index)