The filterdemo computes a FIR lowpass filter by solving a linear program.  
It solves a discretization of the  problem

minimize     d2
subject to   -1/d1 <= H(wk) <= d1,   0 <= w <=  wc
	     max(abs(H(wk))) <= d2,  ws <= w <= pi

where H(w) = h_0 + sum_{i=1}^{n-1} h_i*cos(2*pi/n*i*w).
The variables are h and d2.

'filterdemo_cli' uses a simple a simple command-line interface. 
'filterdemo_gui' uses a graphical user interface based on GTK.
