input: Write a python function to measure elapsed time using the Stopwatch class.
  ```python
  class Stopwatch:
      def __init__(self):
          self._start_time = None
          self._end_time = None
  
      def start(self):
          self._start_time = time.time()
  
      def stop(self):
          self._end_time = time.time()
  
      def get_elapsed_time(self):
          return self._end_time - self._start_time
  ```
  assert elapsed_time_example_1() == (approximately the expected elapsed time)
  assert elapsed_time_example_2() == (approximately the expected elapsed time)
output: import time
  
  def elapsed_time_example_1():
      stopwatch = Stopwatch()
      stopwatch.start()
      time.sleep(1)  # Simulating some task that takes 1 second
      stopwatch.stop()
      return stopwatch.get_elapsed_time()
  
  def elapsed_time_example_2():
      stopwatch = Stopwatch()
      stopwatch.start()
      time.sleep(2)  # Simulating some task that takes 2 seconds
      stopwatch.stop()
      return stopwatch.get_elapsed_time()