code: function step (pointsToEvaluate, functionValuesX, functionValuesY) {
    return help.makeItArrayIfItsNot(pointsToEvaluate).map(function (point) {
      return functionValuesY[help.findIntervalLeftBorderIndex(point, functionValuesX)]
    })
  }
docs: Evaluates interpolating step function at the set of numbers
  or at a single number
  
  @param {Number|Array} pointsToEvaluate     number or set of numbers
  for which step is calculated
  @param {Array} functionValuesX             set of distinct x values
  @param {Array} functionValuesY             set of distinct y=f(x) values
  @returns {Array}