dwidenoise: Alternative noise level estimation

[Lestropie]

Suggested by @bjeurissen.

https://ieeexplore.ieee.org/document/6846297

Separate thresholds for a priori known vs. unknown noise level; the former is unlikely to be usable in practise since we don't typically have independent data from which to estimate the noise level.

It also presumably applies only to hard thresholding, whereas in 3.1.0 the default will be optimal shrinkage; I don't know whether computing a threshold based on optimality of it being a hard threshold, then doing optimal shrinkage, is mathematically appropriate.

[Lestropie]

Added in b772559 as part of #3029.