corr(ds: Series): number {
    if (!(ds instanceof Series))
      throw new Error('ds must be a Series');

    if (ds.length !== this.length)
      throw new Error('Series must be of equal length');

    return this.cov(ds) / (this.std() * ds.std());
  }
Calculate the correlation between this `Series` and another `Series` or iterable

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

@param {Series} ds
   Series with which to calculate correlation

@returns {number}

@example
const ds1 = new Series([1, 2, 3, 4, 5]);
const ds2 = new Series([2, 4, 6, 8, 10]);

// Returns 1
ds1.corr(ds2);

// Also returns 1
ds2.corr(ds1);