Update docstrings with examples in frame

Author: risenW

src/danfojs-base/aggregators/groupby.ts

@@ -1,6 +1,20 @@
+/**
+*  @license
+* Copyright 2021, JsData. All rights reserved.
+*
+* This source code is licensed under the MIT license found in the
+* LICENSE file in the root directory of this source tree.
+
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+* ==========================================================================
+*/
 import DataFrame from "../core/frame"
 import { ArrayType1D, ArrayType2D } from "../shared/types"
-import { variance, std, median, mode, mean } from 'mathjs';
+import { variance, std, median, mode } from 'mathjs';
 import  concat from "../transformers/concat"
 import Series from "../core/series";
 

src/danfojs-base/core/daterange.ts

@@ -1,3 +1,17 @@
+/**
+*  @license
+* Copyright 2021, JsData. All rights reserved.
+*
+* This source code is licensed under the MIT license found in the
+* LICENSE file in the root directory of this source tree.
+
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+* ==========================================================================
+*/
 import Utils from "../shared/utils";
 
 const utils = new Utils();