input: 
  Write a function to calculate the average value of elements in a 2-dimensional array. 
  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]])==5.55
  assert averageMatrix([[1,2],[3,4]])==2.5
  assert averageMatrix([[10],[20],[30]])==20.0
  assert averageMatrix([[0],[0]])==0.0
  
output:     count = 0
      for row in matrix:
          count += len(row)