input: Write a function to read an array from the user and then determine the mean and median of the array. Return the mean and median as a tuple.
  assert mean_median([10, 15, 20, 25, 30, 35])==(22.5, 22.5)
  assert mean_median([1, 2, 3, 4, 5])==(3.0, 3)
  assert mean_median([100, 200, 300, 400, 500])==(300.0, 300)
  assert mean_median([7, 3, 1, 4, 1, 5, 9])==(4.285714285714286, 4)
  assert mean_median([])==(None, None)
output: def mean_median(arr):
      if not arr:
          return (None, None)
      arr.sort()
      mean = sum(arr) / len(arr)
      n = len(arr)
      if n % 2 == 0:
          median = (arr[n//2 - 1] + arr[n//2]) / 2
      else:
          median = arr[n//2]
      return (mean, median)