ANTIALIAS - Butterworth anti-aliasing filter

antialias	use before increasing the sampling interval of data
		 i.e. subsampling 

Function Prototype:
void antialias (float frac, int phase, int n, float p[], float q[]);

Input:
frac		current sampling interval / future interval (should be <= 1)
phase		=0 for zero-phase filter; =1 for minimum-phase filter
n		number of samples
p		array[n] of input samples

Output:
q		array[n] of output (anti-alias filtered) samples		

Notes:
The anti-alias filter is a recursive (Butterworth) filter.  For zero-phase
anti-alias filtering, the recursive filter is applied forwards and backwards.

Author:  Dave Hale, Colorado School of Mines, 06/06/90