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]])==4.77
  assert averageMatrix([[1,2],[3,4]])==2.5
  assert averageMatrix([[5]])==5
  assert averageMatrix([[]])==0
  assert averageMatrix([])==0
output: def averageMatrix(matrix):
      if not matrix:
          return 0
      total = 0
      count = 0
      for row in matrix:
          for element in row:
              total += element
              count += 1
      return total / count if count != 0 else 0