A first cut at a port of the FastAI V2 API to Julia

This code is inspired by FastAI, but differs in implementation in several ways. Most importantly, the original Python code makes heavy use of side-effects where the Learner holds different state variables, and other objects access and modify them.

This has been replaced by a more functional design. The state is now transmitted via arguments to Callbacks which may then pass them on to Metrics

Much of the documentation has been copied from the original Python, and modified where appropriate

The original source is here

https://github.com/fastai/fastai2/blob/master/fastai2/

The documentation is copied from here

https://github.com/fastai/fastai2/blob/master/docs/