Song Six aftertouch is now mapped to volume (buggy)

The volume value sent to the output device  is
64 + half the input aftertouch value. This means that
rests in the controller should be played at (or near)
volume=64.
Bug Fix: The E-MU Modulation wheel now works correctly.
This all works very well, but there is now a KNOWN BUG:
Some of Clytemnestra's syllables are now not sounding.
For example: Her very last "Calls". This does not sound even
when starting the performance there (even though the winds
are sounding)... Is the volume correct when the syllable is
sent?
Bug fix: see previous commit (and comment here) (still buggy)

Apart from the bug mentioned in the previous commit,
the running marker was not moving to the final rest in track 0
of Song Six. This bug has been fixed.
Also, tracks that have no moments between the start and end
markers are now handled correctly.
Fixing these bugs also seems to have fixed the problem with
some of Clytemnestra's syllables not sounding in assisted
performances, but assisted performances have become
rather uncontrollable -- needs investigting.
Simplified assistant.playSpan(). (no more known bugs)

Setting the assistant's performedSequences array in assistant.playSpan(...) has been considerably simplified.
This was possible because the msPositions of the start
and end markers are now much more restricted than
they used to be (some time ago). This simplification was
long overdue.
Also did some refactoring similar to that already done in Sequence.js (Using startMarkerMsPosition and
endMarkerMsPosition make it clearer what these values
actually are.).
The uncontrollability of performance has been much improved by raising the "Retrigger" value in the R2M
(ribbon to MIDI controller) to 80ms (from 0!).