Kern fixes (parsing editorials)

[manoskary]

Support for parsing Kern Files with Editorial Comments and Alternative Passages.

This PR addresses the parsing of Kern scores that include editorial comments such as alternative passages.
Such passages that could previously cause parsing errors can now be ignored and only the original passage or part is kept.

Specific Comments on Code

• Two lines are commented out in load_kern which were inverting the part order, this has to be avoided in case the far right part contains only editorials. Instead, the part order is inversed when the partlist is passed to the score. Since this functionality changes, it should be monitored for potential errors.
• A Kern Object class is introduced for elements that do not correspond to partitura objects but describe the Kern parsing process within the format (e.g. editorials, or voice splitting/merging)