Introduced BeatPath/BeatPathSet class to abstract.

Abstracted the list manipulation so that we can be more flexible when dealing with lilypond score generation. Old way only one list of BeatPaths elements was given as input, but now we want a list of lists of BeatPaths, thus this approach. Better safe than sorry.



Signed-off-by: parhamfh <parhamfh@kth.se>
bugfix: Ignored index to BeatPath __init__

Signed-off-by: parhamfh <parhamfh@kth.se>
Improved ugly way of resetting BeatPair index counter.



Signed-off-by: parhamfh <parhamfh@kth.se>