refactor DPP & DVM

contracts/lib/ExternalValue.sol

@@ -0,0 +1,29 @@
+/*
+
+    Copyright 2020 DODO ZOO.
+    SPDX-License-Identifier: Apache-2.0
+
+*/
+
+pragma solidity 0.6.9;
+pragma experimental ABIEncoderV2;
+
+import {Ownable} from "./Ownable.sol";
+
+interface IExternalValue {
+    function set(uint256) external;
+
+    function get() external view returns (uint256);
+}
+
+contract ExternalValue is IExternalValue, Ownable {
+    uint256 public _VALUE_;
+
+    function set(uint256 value) external override {
+        _VALUE_ = value;
+    }
+
+    function get() external override view returns (uint256) {
+        return _VALUE_;
+    }
+}