input: Write a function to calculate the average of the 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]))
  
  a = [[3,2,6],
       [8,5,9],
       [4,7,1]]
  
  print(averageMatrix(a)) # 4.77
  
  a = []
  print(averageMatrix(a)) # 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]))