Update BWV988 v15

update to LilyPond v2.18.0
Bar 4: fix slurs
Bar 5: add slur to bass
Bar 8: top voice change slurred d8's to tied
Bar 9: fix slur
Bar23: change slurred bes2 to tied
Bar25: Add slur
Bar30: Add Tie
Adjust beams, staff voice changes, rest positions
Set midi tempo
(Javier Ruiz-Alma)
Close #97