input: Write a function to compute the average of elements in a 2-dimensional array. If the array is empty, return 0.
  assert averageMatrix([[3,2,6], [8,5,9], [4,7,1]])==5.22
  assert averageMatrix([[1,2],[3,4]])==2.5
  assert averageMatrix([[0,0],[0,0]])==0
  assert averageMatrix([])==0
  assert averageMatrix([[5]])==5
output: def averageMatrix(matrix):
      if not matrix:
          return 0
      total = 0
      count = 0
      for row in matrix:
          total += sum(row)
          count += len(row)
      return total / count