sum(): number {
    return sum(this.values);
  }
Return the sum of the values in the `Series`

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

@returns {number}

@example
const ds = new Series([1, 2, 3, 4], {name: 'New Series'})

// Returns 10
ds.sum();