scurve - special generator for declicking beginnings and ends of notes

	gen T scurve 

scurve generates a sequence of values that rise from 0 to 1 during the
first third of the sequence, and fall from 1 to 0 during the last third.
The middle third of the sequence has a constant value of 1.  The shape
of the rise and fall is a maximally smooth "S-curve" with sinusoidal shape.
The resulting sequence is ideally suited for use as a 3-segment envelope
with the seg unit generator to remove clicks from the beginnings and ends
of notes.