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

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

@returns {number}

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

// Returns 2.5
ds.mean();