input: Write a python function to find the maximum subarray sum.
  assert maximum_sum([1, -2, 3, 4, -1, 2, 1, -5, 4], 9) == 10
  assert maximum_sum([5, -3, 5], 3) == 10
  assert maximum_sum([-1, -2, -3, -4], 4) == 0
output: def maximum_sum(arr, n):
      max_so_far = 0
      max_ending_here = 0
      
      for i in range(n):
          max_ending_here = max_ending_here + arr[i]
          if max_ending_here < 0:
              max_ending_here = 0
          elif max_so_far < max_ending_here:
              max_so_far = max_ending_here
      
      return max_so_far