Additive white gaussian noise (AWGN) generation

# What does it do?

Adding noise is not the most useful thing in most DSP applications, but it is awfully useful for test suites.

# How does it work?

This code is based on some demonstration code in a research paper somewhere. I can't track down where I got the original from, so that due recognition can be given. The original had no explicit copyright notice, and I hope nobody objects to its use here.

Having a reasonable Gaussian noise generator is pretty important for telephony testing (in fact, pretty much any DSP testing), and this one seems to have served me OK. Since the generation of Gaussian noise is only for test purposes, and not a core system component, I don't intend to worry excessively about copyright issues, unless someone worries me.

The non-core nature of this code also explains why it is unlikely to ever be optimised.