[OHIF-177] - JSDocs (#1818)

* OHIF-177 JSDocs.

* Add proxy for activeDataSource to extensionManager.

platform/core/src/extensions/ExtensionManager.js

@@ -137,6 +137,7 @@ export default class ExtensionManager {
 
   getDataSources = dataSourceName => {
     if (dataSourceName === undefined) {
+      // Default to the activeDataSource
       dataSourceName = this.activeDataSource;
     }
 
@@ -144,6 +145,10 @@ export default class ExtensionManager {
     return this.dataSourceMap[dataSourceName];
   };
 
+  getActiveDataSource = () => {
+    return this.activeDataSource;
+  };
+
   /**
    * @private
    * @param {string} moduleType