ms-challenge-2021 v0.2

Second release of the ms-challenge-2021 package. Contains the following changes:

1. Minor clarifications on the preprocessing script and method.
2. Added implementations of Attention U-Net and Monte-Carlo Dropout for model resembling
3. ReadMe updates, notably linking our paper on ArXiv.

NOTE: This release corresponds to NeuroPoly lab's submission for the MICCAI 2021 MS New Lesions Segmentation Challenge.