Clean up the database, e.g. removing unused keywords.

def cleanup(self):
        ''' Clean up the database, e.g. removing unused keywords.'''
        allList = []
        allList.extend(self.cur.execute("SELECT keywordid FROM keyword;"))
        usedList = []
        usedList.extend(self.cur.execute("SELECT keywordid FROM notekeyword;"))
        unusedList = [val for val in allList if val not in usedList]
        for key in unusedList:
            if self.debug:
                print("About to delete keyword with ID %s" % key)
            try:
                self.cur.execute("DELETE FROM keyword WHERE keywordId = ?;", key)
            except:
                self.error("There was a problem deleting keyword %s" % key)
        self.con.commit()