pairIsInCorrectOrder(firstElement, secondElement) {
    throw new Error(`
      You have to implement heap pair comparison method
      for ${firstElement} and ${secondElement} values.
    `);
  }
Checks if pair of heap elements is in correct order.
For MinHeap the first element must be always smaller or equal.
For MaxHeap the first element must be always bigger or equal.

@param {*} firstElement
@param {*} secondElement
@return {boolean}