input: Write a python function to retrieve a student by their student_id from the Database class.
  assert retrieve_student(database, 1).name == 'Alice'
  assert retrieve_student(database, 2).name == 'Bob'
  assert retrieve_student(database, 3) is None
output: def retrieve_student(database, student_id):
      for student in database.students:
          if student.student_id == student_id:
              return student
      return None