Add `InkMottling` Augmentation to Ensure Ink is Non-Uniform

[jboarman]

To render real-world documents, ink sourced from a perfect PDF source should not render as perfect black, for example. Such examples are easily denoised with threshold filters. So we need to add noise to the incoming ink layer.

Effects of InkMottling Augmentation

• [ ] The existing ink layer is adjusted by a noise layer
• [ ] The noise could be composed of positive and negative numbers (useful when ink is not strictly black or white) so that levels are adjusted both up and down
• [ ] Noise options would start with fast noise options like Poisson or Gaussian or Speckle noise (see deprecated code from skimage)
• [ ] The range of noise values effecting ink should be parameterized -- by default, this range should be low and subtle, often un-noticeable to the human eye unless you are looking for it
• [ ] The noise layer should randomly apply to either all channels, or be independent noise applied separately to each channel (to simulate black rendered by color ink)

Example Problem Image (what this augmentation will fix)

Example Real-World Mottling of Black Ink

[kwcckw]

This is now added in this pull request: https://github.com/sparkfish/augraphy/pull/305