gen3 - simple line segment function generator similar to the MUSIC V GEN 3:

	gen T gen3 F  v1 v2 ... vN ;

The arbitrarily long list of values v1, ..., vN specify relative amplitude
at equally spaced points along a (closed) function. Thus

	gen 0 gen3 f2    0 1 1 0 ;

specifies a trapezoidal function which has 3 parts: the first rises in a
straight line from 0 to 1, the second is steady at 1, and the third falls
from 1 to 0.  If all values are positive, the function is scaled to be 
only positive.  All negative values result in an all negative function.  
Values may be both positive and negative, resulting in a function which 
ranges between +1 and -1.
