input: Write a python function to perform sentiment analysis using NLTK's Vader tool.
  assert sentiment_analysis("I'm feeling really great today") == 0.6249
  assert sentiment_analysis("This is the worst day ever") < 0
  assert sentiment_analysis("I have mixed feelings about this") == 0
  assert sentiment_analysis("It's an average day") > 0
output: def sentiment_analysis(text):
      from nltk.sentiment.vader import SentimentIntensityAnalyzer
      sia = SentimentIntensityAnalyzer()
      score = sia.polarity_scores(text)['compound']
      return score