copy(): DataFrame {
    return new DataFrame(this._data, {index: this.index});
  }
Return a new deep copy of the `DataFrame`

pandas equivalent: [DataFrame.copy](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.copy.html)

@returns {DataFrame}

@example
const df = const df = new DataFrame([{x: 1, y: 2}, {x: 2, y: 3}, {x: 3, y: 4}]);
const df2 = df.copy();
df2.index = [1, 2, 3];
df.index   // [0, 1, 2];
df2.index  // [1, 2, 3];