input: Write a function to calculate the average of the elements in a 2-dimensional array. The function should return the average as a floating-point number.
  assert averageMatrix([[3,2,6], [8,5,9], [4,7,1]])==4.77
  assert averageMatrix([[1,2], [3,4]])==2.5
  assert averageMatrix([[0,0], [0,0]])==0.0
  assert averageMatrix([[10,20], [30,40]])==25.0
  assert averageMatrix([[1], [2], [3]])==2.0
output: def averageMatrix(matrix):
      total = 0
      count = 0
      for row in matrix:
          total += sum(row)
          count += len(row)
      return total / count