tuning opcode (and/or header)

[alcomposer]

Both sfizz and Sforzando are able to load Scala tuning files in UI.
Some instruments may need a tuning file in order to function correctly.

For these instruments an opcode that points to an included tuning file would be needed:

<region> sample=*sine tuning=mytuning.scl

Optionally tuning may be defined in SFZ itself, similar to curve:

<tuning>
tuning_index=1
i1= 76.04900
i2=193.15686
i3=310.26471
i4=5/4
i5=503.42157
i6=579.47057
i7=696.57843
i8=25/16
i9=889.73529
i10=1006.84314
i11=1082.89214
i12=2/1

As scala allows both decimal (cent), and fraction (ratio) definition of intervals (i) so to should <tuning>

The UI of said players should still be able to override the SFZ script tuning, if needed by the user.