opendaylight-flow-types.yang is invalid

opendaylight-flow-types contains an invalid augmentation of opendaylight-multipart-types.yang, specifically:

    augment "/multipart:multipart-request/multipart:multipart-request-body" {
        case multipart-request-flow-stats {
            uses flow;
        }
        case multipart-request-flow-aggregate-stats {
            uses flow;
        }
    }

This ends up introducing duplicate containers/leaves across the cases, which is explicitly forbidden by RFC7950: https://tools.ietf.org/html/rfc7950#section-7.9.2

This needs to be resolved before we can integrate yangtools-2.0.7.