usage: w = gaussian(n, a)

Generate an n-point gaussian convolution window of the given
width.  Use larger a for a narrower window.  Use larger n for
longer tails.

w = exp ( -(a*x)^2/2 )

for x = linspace ( (n-1)/2, (n-1)/2, n ).

Width a is measured in frequency units (sample rate/num samples).
It should be f when multiplying in the time domain, but 1/f when
multiplying in the frequency domain (for use in convolutions).