Inner Class challenge based on https://github.com/Csaba79-coder/PlaylistForSongs
// Modify the playlist challenge so that the model.Album class uses an inner class.
// Instead of using an ArrayList to hold its tracks, it will use an inner class called SongList
// The inner SongList class will use an ArrayList and will provide a method to add a song.
// It will also provide findSong() methods which will be used by the containing model.Album class
// to add songs to the playlist.
// Neither the model.Song class nor the Main class should be changed.