Example of morphing (interpolating) between two Signals/Wavetables

This is in response to a question on sc-users, with subject: [Envelope as signal](http://new-supercollider-mailing-lists-forums-use-these.2681727.n2.nabble.com/Envelope-as-signal-td7603680.html), about how to morph between two different Envelopes-as-wavetables... I tried to do one with double-buffering and cross-fading between the two buffers using XFade2.ar, but it gave me a sort of "pulsing" effect. You even get this when doing XFade2 between two versions of the same signal! But, fortunately, single-buffering seems to work fine. The changes to any given sample in the Signal/Wavetable are small at any iteration, so you don't hear any noise or clicking. (you would if you had too few steps in the interpolation, but even with 20-50 steps, it seems noise-free)