Add thresholding algorithms submodule

[jmetz]

Ideas for algorithms to add could come from Matlab/Scikit-image/ImageJ threshold algorithms.

To start we could add some a couple of standard ones - I've been working on mean and Otsu's method implementations.

[xd009642]

@jmetz are there any other thresholding algorithms you'd like in particular?

[jmetz]

I use Scikit Image a lot - and often use e.g Li threshold algorithm (https://github.com/scikit-image/scikit-image/). In principle though I could imagine implementing all of the standard threshold algorithms as a long term goal.

ImageJ also contains a good selection of threshold algorithms to pick from: https://imagej.net/Auto_Threshold

[xd009642]

Yeah I've got no issue with implementing them all it's just a matter of time (or getting the PRs 😄 )