code: function meanAbsoluteDeviation(actuals, estimates) { 
    const errors = forecastErrors(actuals, estimates).map(e=>Math.abs(e));
    return avg(errors);
  }
docs: Mean Absolute Deviation (MAD) indicates the absolute size of the errors
  @memberOf util
  @see {@link https://scm.ncsu.edu/scm-articles/article/measuring-forecast-accuracy-approaches-to-forecasting-a-tutorial}
  @example
  const actuals = [ 45, 38, 43, 39 ];
  const estimates = [ 41, 43, 41, 42 ];
  const MAD = ms.util.meanAbsoluteDeviation(actuals, estimates); // =>  3.5
  @param {Number[]} actuals - numerical samples
  @param {Number[]} estimates - estimates values
  @returns {Number} MAD