input: Write a function to calculate the average of elements in a 2-dimensional array. If the given 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]])==4.77
  assert averageMatrix([])==0
  assert averageMatrix([[1,1,1], [1,1,1]])==1.0
  assert averageMatrix([[0,0], [0,0]])==0.0
  assert averageMatrix([[5,5], [5,5]])==5.0
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]))