input: Write a function to calculate the average of elements in a 2-dimensional array. If the array is empty, return 0.
  def averageMatrix(matrix):
   if not matrix:
    return 0
   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]]) == 5.0
  assert averageMatrix([[1,2], [3,4]]) == 2.5
  assert averageMatrix([[0,0]]) == 0
  assert averageMatrix([]) == 0
output: def averageMatrix(matrix):
   if not matrix:
    return 0
   sum = 0
   for row in matrix:
    for e in row:
     sum += e
  
   return sum / (len(matrix) * len(matrix[0]))