input: Write a function to compute the average of elements in a 2-dimensional array. The function should return the average value.
  
  def averageMatrix(matrix):
      sum = 0
      for row in matrix:
          for e in row:
              sum += e
      return sum / (len(matrix) * len(matrix[0]))
  
  assert averageMatrix([[3,2,6],[8,5,9],[4,7,1]])==4.77
  assert averageMatrix([[1,2],[3,4]])==2.5
  assert averageMatrix([[10,20],[30,40]])==25.0
  assert averageMatrix([[1,1],[1,1]])==1.0
  assert averageMatrix([[]])==None
output: def averageMatrix(matrix):
      if not matrix or not matrix[0]:
          return None
      sum = 0
      count = 0
      for row in matrix:
          for e in row:
              sum += e
              count += 1
      return sum / count