Added groupby.std()

Author: Devwulf

src/danfojs-base/aggregators/groupby.ts

@@ -441,6 +441,14 @@ export default class Groupby {
     return this.operations("sum")
   }
 
+  /**
+   * Obtain the standard deviation of columns for each group
+   * @returns DataFrame
+   */
+  std(): DataFrame{
+    return this.operations("std")
+  }
+
   /**
    * Obtain the variance of columns for each group
    * @returns DataFrame