Macenko stain augmentation

[andreped]

Related to issue https://github.com/EIDOSLAB/torchstain/issues/43.

Changes

• I implemented StainTools-like Macenko augmentation for all three backends.
• It also made a sense to implement an augmentation example script, and thus I moved both examples to their own dedicated apps/.
• Due to this, I changed to run tests on macos-12 instead of macos-11
• I also fixed a step in the tests CI which failed due to breaking change in artifact download

Ideas from improvement

API-wise I believe there is a lot of overlap between the normalization and augmentation techniques. It would make sense to have a Macenko ABC class, of which the MacenkoNormalizer and MacenkoAugmentor classes inherit from, and then for each we just override the necessary methods.