code: function standardScore(observations = []) {
                const mean = avg$2(observations);
                const stdDev = sd(observations);
                return observations.map(x => ((x - mean) / stdDev));
              }
docs: Calculates the z score of each value in the sample, relative to the sample mean and standard deviation.
  @memberOf util
  @see {@link https://docs.scipy.org/doc/scipy-0.14.0/reference/generated/scipy.stats.mstats.zscore.html}
  @param {Number[]} observations - An array like object containing the sample data.
  @returns {Number[]} The z-scores, standardized by mean and standard deviation of input array