input: Write a python function to find the maximum sum subarray in a given list.
  assert maximum_subarray([1, -2, 3, 4, -1, 2, 1, -5, 4]) == 9
  assert maximum_subarray([-2, -3, 4, -1, -2, 1, 5, -3]) == 7
  assert maximum_subarray([1, 2, 3, 4, 5]) == 15
output: def maximum_subarray(nums):
      max_sum = nums[0]
      current_sum = nums[0]
      
      for num in nums[1:]:
          current_sum = max(num, current_sum + num)
          max_sum = max(max_sum, current_sum)
      
      return max_sum