input: Write a python function to implement a max priority queue with methods to add elements, delete the maximum element, and maintain the heap property.
  assert pq.add(50) == None
  assert pq.delMax() == 105
  assert pq.delMax() == 62
  assert pq.delMax() == 50
output: import heapq
  
  class MaxPriorityQueue:
      def __init__(self):
          self._heap = []
      
      def add(self, val):
          heapq.heappush(self._heap, -val)
      
      def delMax(self):
          if not self._heap:
              return None
          return -heapq.heappop(self._heap)
          
  pq = MaxPriorityQueue()
  pq.add(105)
  pq.add(62)
  pq.add(50)