Update Bach BWV988 var21

update to LilyPond v2.18.2
Bar 3: 4th beat 2nd voice b16->bes16
Bar 5: 2nd beat missing tie bes8~bes16
Bar 6: Downprall on 1st voice moved to 3rd beat, 2nd beat remove beam to 2nd voice quarter notes
Bar10: 2nd voice 4th beat missing tie on aes4~aes16
Bar13: 2nd beat 2nd voice ees->e
Adjust voice staff placement and beaming to match source
Add courtesy accidentals
Set midi tempo
(Javier Ruiz-Alma)
Close #102