Attack-Decay-Sustain-Release Envelope. If *done* boolean array flag is used,
will await until done is set to true before performing release stage.  Otherwise, 
defaults to *duration* or 1.0 for total time of envelope.

Based on code by Nigel Redmon at http://www.earlevel.com/main/2013/06/03/envelope-generators-adsr-code/

ADSR Envelope generator based on Doepfer A-140. Takes in gate audio
function, as well as retrigger audio function (pass in 0 if not using
retrigger). 

Based on code by Nigel Redmon at
http://www.earlevel.com/main/2013/06/03/envelope-generators-adsr-code/

Generates an envelope given pairs of values (t0, v0, t1, v1 ...) where tx is duration of segment.

Generates an exponential envelope given pairs of values (t0, v0, t1, v1 ...) where tx is duration of segment.

Simple envelope that holds a given value for a given
duration.  Will zero out after duration if duration ends
mid-buffer, then return nil afterwards to signal
completion.

Line segment generator using formula for each segment:

ibeg +  (ivalue - ibeg) * (1 - exp( i*itype/(n-1) )) / 
    (1 - exp(itype))

for n steps. Points are given in format of:

ia, idur, itype, ib, idur2, itype2, ic...

where i_ is a value, dur is time in seconds, and itype affects the curve of the segment (0 = straight line, < 0 = conve, > 0 = concave))

Exponential Attack-Release Envelope