std(): number {
    return Math.sqrt(this.variance());
  }
Return the standard deviation of the values in the `Series`

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

@returns {number}

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

// Returns 1
ds.std();