input: Write a function to calculate the average of elements in a 2-dimensional array. The function should take a 2D matrix as input and return the average value of all elements in the matrix.
  assert averageMatrix([[3, 2, 6], [8, 5, 9], [4, 7, 1]])==5.0
  assert averageMatrix([[1, 2], [3, 4]])==2.5
  assert averageMatrix([[5]])==5.0
  assert averageMatrix([[], []])==0.0
  assert averageMatrix([[1, 1, 1], [1, 1, 1]])==1.0
output: def averageMatrix(matrix):
      total = 0
      count = 0
      for row in matrix:
          for element in row:
              total += element
              count += 1
      if count == 0:
          return 0.0
      return total / count