code: def get_user(self, user_id):
          
          if user_id in self._users:
              return self._users.get(user_id)
          else:
              # Load user
              # Save user in cache
              return
docs: Details for a specific user.
          Will pull from cached users first, or get and add to cached users.
          :param username: the username or userId of the user
          :return: VoicebaseUser