implement original MRNet in fastai

[wfwiggins]

1. mrnet_orig.py adds classes for loading data and creating a custom Learner for the MRNet model with a custom weighted binary cross-entropy loss function (due to the class imbalance in the MRNet data)
2. MRNet_fastai_example.ipynb demonstrates example usage of the classes in (1)

Note: I have yet to implement data augmentation due to conflict between the need for image stacks and my forcing their implementation via the fast.ai Image class.

[review-notebook-app]

Check out this pull request on ReviewNB: https://app.reviewnb.com/lessw2020/mrnet-fastai/pull/8

Visit www.reviewnb.com to know how we simplify your Jupyter Notebook workflows.