CONVOLUTION - Compute z = x convolved with y

conv	compute the convolution of two input vector arrays

Input:
lx		length of x array
ifx		sample index of first x
x		array[lx] to be convolved with y
ly		length of y array
ify		sample index of first y
y		array[ly] with which x is to be convolved
lz		length of z array
ifz		sample index of first z

Output:
z		array[lz] containing x convolved with y

Function Prototype:
void conv (int lx, int ifx, float *x, int ly, int ify, float *y,
	int lz, int ifz, float *z);

Notes:
The operation z = x convolved with y is defined to be
           ifx+lx-1
    z[i] =   sum    x[j]*y[i-j]  ;  i = ifz,...,ifz+lz-1
            j=ifx
The x samples are contained in x[0], x[1], ..., x[lx-1]; likewise for
the y and z samples.  The sample indices of the first x, y, and z values
determine the location of the origin for each array.  For example, if
z is to be a weighted average of the nearest 5 samples of y, one might
use 
	...
	x[0] = x[1] = x[2] = x[3] = x[4] = 1.0/5.0;
	conv(5,-2,x,lx,0,y,ly,0,z);
	...
In this example, the filter x is symmetric, with index of first sample = -2.

This function is optimized for architectures that can simultaneously perform
a multiply, add, and one load from memory; e.g., the IBM RISC System/6000.
Because, for each value of i, it accumulates the convolution sum z[i] in a
scalar, this function is not likely to be optimal for vector architectures.

Author:  Dave Hale, Colorado School of Mines, 11/23/91