get dtype(): DType {
    return this._dtype;
  }
Return the dtype of the underlying data

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

@returns {DType}

@example
const ds = new Series([1.5, 2, 3], {name: 'Series name'});
ds.dtype;    // dtype('float');