Common DSP tools

* dB conversions, e.g. `x = -3dBa*x` for decreasing the power of `x` by 2 if it is an amplitude signal. For scaling powers, use `p = -3dB*p`.
* rms
* snr


There are some common functions in https://github.com/JuliaStats/StatsBase.jl/blob/master/src/deviation.jl for DSP, so maybe the `snr` function fits better there?