code: function avg(arr) {
      arr = arr || [];
      if (arr.length) {
          var total = sum(arr);
          if (number.isNumber(total)) {
              return total / arr.length;
          } else {
              throw new Error("Cannot average an array of non numbers.");
          }
      } else {
          return 0;
      }
  }
docs: Averages an array of numbers.
  @example
  
  comb.array.avg([1,2,3]); //2
  
  @param {Number[]} array - an array of numbers
  @return {Number} the average of all the numbers in the array.
  @throws {Error} if the array is not all numbers.
  @static
  @memberOf comb.array