Add MLCubes for evaluation

[mzenk]

This PR gathers all MLCubes required for running the evaluation of task 2 in the official challenge repository (previously distributed among medperf repo and mlcube_examples repo). At the same time, it also updates these cubes so that they use the challenge I/O conventions.

This is still WIP.

• [x] Make the BraTS MedPerf example compatible with the FeTS conventions
• [x] Test individual MLCubes
• [x] Run MedPerf pipeline (compatibility test)
• [x] Add documentation how to run compatibility test for participants
• [x] (optional) Add logging to data preparation cube

[mzenk]

@sarthakpati @ujjwalbaid0408 would be great if one of you could merge this today.