input: Write a python function to calculate the sum of all numbers from 0 to n recursively.
  assert sum_recursive(10) == 55
  assert sum_recursive(5) == 15
  assert sum_recursive(0) == 0
output: def sum_recursive(n):
      if n == 0:
          return 0
      else:
          return n + sum_recursive(n - 1)