change the echarts import mode

docs/zh/api/global.md

 ```json
 //属性参数（可选）
 {
-  "baseUrl": "<Cesium 静态资源路径，默认值为：“./libs/dc-sdk/resources/” >"
+  "baseUrl": "<Cesium 静态资源路径，默认值为：“./libs/dc-sdk/resources/” >",
+  "echarts": "<echarts库，设置后用于ChartLayer>"
 }
 ```
 

examples/echarts/airline.html

     <div id="viewer-container" class="viewer-container"></div>
 
     <script>
-      DC.registerLib('echarts', window.echarts)
       let viewer = undefined
       function initViewer() {
         viewer = new DC.Viewer('viewer-container')
 
       DC.ready({
         baseUrl: '../libs/dc-sdk/resources/',
+        echarts: window.echarts,
       }).then(initViewer)
     </script>
   </body>

examples/echarts/logistics.html

     <div id="viewer-container" class="viewer-container"></div>
 
     <script>
-      DC.registerLib('echarts', window.echarts)
       let viewer = undefined
       function initViewer() {
         viewer = new DC.Viewer('viewer-container')
 
       DC.ready({
         baseUrl: '../libs/dc-sdk/resources/',
+        echarts: window.echarts,
       }).then(initViewer)
     </script>
   </body>

examples/echarts/migrate.html

     <div id="viewer-container" class="viewer-container"></div>
 
     <script>
-      DC.registerLib('echarts', window.echarts)
       let viewer = undefined
       function initViewer() {
         viewer = new DC.Viewer('viewer-container')
 
       DC.ready({
         baseUrl: '../libs/dc-sdk/resources/',
+        echarts: window.echarts,
       }).then(initViewer)
     </script>
   </body>

examples/echarts/plane.html

     <div id="viewer-container" class="viewer-container"></div>
 
     <script>
-      DC.registerLib('echarts', window.echarts)
       let viewer = undefined
       function initViewer() {
         viewer = new DC.Viewer('viewer-container')
 
       DC.ready({
         baseUrl: '../libs/dc-sdk/resources/',
+        echarts: window.echarts,
       }).then(initViewer)
     </script>
   </body>

examples/echarts/pm.html

     <div id="viewer-container" class="viewer-container"></div>
 
     <script>
-      DC.registerLib('echarts', window.echarts)
       let viewer = undefined
       function initViewer() {
         viewer = new DC.Viewer('viewer-container')

examples/echarts/population_mobility.html

     <div id="viewer-container" class="viewer-container"></div>
 
     <script>
-      DC.registerLib('echarts', window.echarts)
       let viewer = undefined
       function initViewer() {
         viewer = new DC.Viewer('viewer-container')
 
       DC.ready({
         baseUrl: '../libs/dc-sdk/resources/',
+        echarts: window.echarts,
       }).then(initViewer)
     </script>
   </body>

src/global-api/lib-utils.js

  */
 export function registerLib(name, lib) {
   cache[name] = lib
-  if (name === 'echarts' && this.registerEcharts) {
-    this.registerEcharts(lib)
-  }
-  return this
 }
 
 /**

src/index.js

  @Author: Caven Chen
  **/
 
-import { setParam } from './global-api'
+import { setParam, registerLib } from './global-api'
+
+import { registerEcharts } from './modules/chart'
+
 const DEF_BASE_URL = './libs/dc-sdk/resources/'
 let _baseUrl = DEF_BASE_URL
 let __isInitialized = false
 
 export { Math } from './modules/math'
 
-export { registerEcharts, ChartLayer } from './modules/chart'
+export { ChartLayer } from './modules/chart'
 
 export const config = {
   set baseUrl(baseUrl) {
     return Promise.resolve()
   }
   __cmdOut && __cmdOut()
-  if (options['baseUrl']) {
-    this.config.baseUrl = options['baseUrl']
-  }
-  __isInitialized = true
   return new Promise((resolve, reject) => {
+    //reset CESIUM_BASE_URL
+    if (options['baseUrl']) {
+      this.config.baseUrl = options['baseUrl']
+    }
+    //register echarts lib
+    if (options['echarts']) {
+      registerEcharts(options['echarts'])
+      registerLib('echarts', options['echarts'])
+    }
+    __isInitialized = true
+
     setParam('isInitialized', true)
     setParam('baseUrl', this.config.baseUrl)
     resolve()

src/modules/chart/ChartLayer.js

 
 class ChartLayer extends Layer {
   constructor(id, option) {
+    if (!getLib('echarts')) {
+      throw new Error('miss the echarts lib')
+    }
     super(id)
     this._option = option
     this._delegate = undefined
   }
 
   _onAdd(viewer) {
-    let echarts = getLib('echarts')
-    if (!echarts) {
-      throw new Error('')
-    }
+    const echarts = getLib('echarts')
     this._viewer = viewer
     this._viewer.canvas.setAttribute('tabIndex', '0')
     this._delegate = this._createChartElement()