constructor({ value, weight, itemsInStock = 1 }) {
    this.value = value;
    this.weight = weight;
    this.itemsInStock = itemsInStock;
    // Actual number of items that is going to be added to knapsack.
    this.quantity = 1;
  }
@param {Object} itemSettings - knapsack item settings,
@param {number} itemSettings.value - value of the item.
@param {number} itemSettings.weight - weight of the item.
@param {number} itemSettings.itemsInStock - how many items are available to be added.