More functionality for the Parse class

• Parse.detach_levels() for emptying all parsed scores from annotations
• Parse.store_mscx() for storing altered (e.g. emptied) score objects as MuseScore files
• Parse.metadata() to return a DataFrame with all parsed pieces' metadata
• Parse.get_labels() to retrieve labels of a particular kind
• Parse.info() has improved the information that objects return about themselves
• Parse['key'] for a quick overview of the files of a given key
• Parse can be used with a custom index instead of IDs [an ID is an (key, i) tuple]
• Score.store_list() for easily storing TSVs
• improved expansion of DCML harmony labels