Add unittest for trainer

[kbressem]

This PR adds simple test for the trainer

Solves #51

Changes:

• Test if the trainer can be initialized and runs for one epoch (including validation).
• Test if folder structure and logging files are created correctly.
• Adapt the test data to the correct format. Initially, labels and images were identical. Now labels do represent binary labels.
• Add new requirements to setup.cfg. These were required all along, but because no tests existed, this was not noticed.
• Adapt the post-transforms. This was necessary because of changes in #58 which broke the transforms. Again, this problem was not noticed because of missing tests 🙈. Solves #35
• Adds simple test for new sanity_check in dataloader

[kbressem]

LGTM

I don't understand exactly what happened to the nifty files in data/labels/

Before, the labels were just identical copies of the images. So no real labels. I made them into real binary labels, to allow realistic test conditions.