[sdiy] Update to PIC envelope generator project

[Tom Wiltshire]

Hi all,

I've done an update to my PIC-based VCADSR envelope generator. The  
previous version only had a GATE input, so I've added TRIGGER as  
well. It can still be used with GATE alone if required.

I've also updated the circuit diagram to use an improved two-stage  
bessel filter.

Finally, I changed the TIME CV input that can change the overall time  
of the envelope's A, D and R from a bipolar input to a unipolar one  
that shortens the times. This is more like the SSM2056 which I  
originally used as model.

New code (Version 7B not 7) is now fully tested and online. Enjoy!

Regards,
Tom

(I've been halfway through doing these changes for a week or more,  
but got sidetracked dealing with Internet Explorer problems, now  
sorted.)