Heuristic: Retweet Bot

Heuristic implemented through the method is_retweet_bot of src/bot_detector/heuristics/simple.py that checks the number of retweets in a given user's timeline. The method returns 1 if the ratio between the number of retweets over the number of tweets is greater than or equal to a defined threshold (e.g., 0.90), 0 otherwise. The threshold is defined in the file src/bot_detector/heuristics/heuristic_config.json.