Import for BraTS dataset format

[yasakova-anastasia]

Summary

Depends on #539

• Resolves Import for BraTS dataset format #616
• Added nibabel to requirements
• Reading pickle files is moved to a separate file (also for CIFAR)
• Added a new media format for multiframe images

How to test

Unit tests and manually on original dataset

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[zhiltsov-max]

1. Do you plan to support other tasks from the table in the article?

2. Are pre-2017 and post-2017 versions compatible? If not, we need to clarify the format supported.

[yasakova-anastasia]

Do you plan to support other tasks from the table in the article?

It seems to me that segmentation will be enough, since I haven't seen the use of other tasks somewhere.

Are pre-2017 and post-2017 versions compatible? If not, we need to clarify the format supported.

I supported BraTS'17-20. Added an information about it.