imporve viewer destroy

modules/viewer/Viewer.js

    */
   destroy() {
     this._delegate.destroy()
+    Object.keys(this._layerCache).forEach(type => {
+      let cache = this._layerCache[type]
+      Object.keys(cache).forEach(layerId => {
+        this._removeLayer(cache[layerId])
+      })
+    })
     this._delegate = undefined
+    this._baseLayerPicker = undefined
+    this._layerCache = {}
+    this._dcContainer.parentNode.removeChild(this._dcContainer)
     return this
   }