input: Write a function to find the average of numbers stored in an array. If the array is empty, return 0.
  assert calculate_mean([10, 20, 30])==20.0
  assert calculate_mean([15, 30, 45, 60])==37.5
  assert calculate_mean([5])==5.0
  assert calculate_mean([])==0
output: def calculate_mean(arr):
      if not arr:
          return 0
      return sum(arr) / len(arr)