An implementation of normalized_grid_distortion

[poke1024]

This is a follow up of https://github.com/albumentations-team/albumentations/pull/637 with a new more focused implementation.

In contrast to #637, this implementation is based on the existing grid_distortion, i.e. it does not change its implementation of the num_steps.

Basing this functionality on the existing grid_distortion makes things a bit tricky, as the inner workings of grid_distortion are not super trivial. I've written a rationale of how this implementation comes up with its scaling factors in https://github.com/poke1024/albumentations-normalized-grid/blob/master/albumentations.pdf

I'm not a math whizz, and probably there is some more elegant way to tackle this, but I hope that these explanations make the implementation more clear.

A demo ipynb showing the new NormalizedGridDistortion functionality is here:

https://github.com/poke1024/albumentations-normalized-grid/blob/master/demo.ipynb

As you can see (on the right column at the graphics at the bottom), the new NormalizedGridDistortion will warp the image internally, but never project it outside its bounds. The current default GridDistortion is in the left column.

A standalone version of the notebook (not needing a patched albumentations library) is here: https://github.com/poke1024/albumentations-normalized-grid/blob/master/demo_standalone.ipynb

[poke1024]

In the last PR this thing was implemented as limit_bbox parameter to GridDistortion, now it's a new class NormalizedGridDistortion. I'm not really sure which approach is better. I ended up creating a new class since I started implementing this outside of albumentations and deriving from GridDistortion.