[musicxml2ly] wrong durations in chorded notes with time-modifications

[pls]

Hey all,

Sibelius 7.0.0 exports wrong duration values for chorded notes in tuplets.  The 
exported value in this example 
(​https://www.dropbox.com/s/vrvdabwym9tezd0/time-modification_and_chorded_notes.xml)
 is 256 instead of 170 for a quarter note in a triplet of eighths 
(<divisions>256</divisions>).  <durations>170</durations> is not an optimal 
value for a quarter note in a triplet of eighths as there is a rounding 
difference (256*2/3=170.6666...).  The same holds for an eighth note in a 
triplet of eighths (128*2/3=85.3333... instead of 86 used here).  musicxml2ly 
does not like rounding errors.  The resulting LilyPond-code contains wild 
scaling durations and a superfluous spacer rest: 
\times 2/3 { <c'' a'>4*255/256 s512*43 r8*129/128 } r4 r2 }

This is bad because we have a bar check problem here!

Sometimes music applications export suboptimal/false values for divisions and 
durations.
A solution: musicxml2ly should recalculate all durations on the basis of a 
divisions value of 6720.   This figure is divisible without remainder by e.g. 
2, 3, 4, 5, 6, 7, 8, 16, 32, 64 and therefore can provide for note durations up 
to 256th.  A quarter note (6720) in a triplet of eighths would have a value of 
4480 (6720*2/3).  An eighth note (6720/2=3360) in a triplet of eighths would 
have a value of 2240 (3360*2/3).  (half note: 6720*2 = 13440.)

hth
patrick