smooth - first order lowpass smoothing filter

smooth  output[b] in[bvpn] tau[bvpn] temp[d] ;

The output is a smoothed version of the input according to the lowpass
filter relation:

    y(n) = x(n) + tau*y(n-1)
    output = y(n)*(1 - tau)

The output is scaled by (1-tau) in order to approximate a DC gain of 1.0.
Obviously, tau should normally be less than 1.  The temp location is used
to hold the previous output.
