input: Write a function to calculate the sum of an array of numbers.
  assert sum_array([1, 2, 3, 4, 5]) == 15
  assert sum_array([10, 20, 30, 40]) == 100
  assert sum_array([-1, -2, -3, -4, -5]) == -15
output: def sum_array(arr):
      return sum(arr)