Add unit tests

[MaximLippeveld]

We will use pytest to write unit tests. Some core functionality shoud already be tested in the alpha release.

• [x] Create dataset with 5 multiframe tiff images that can be used for testing (can be pushed to git repo)
• [x] Write data loading test for multiframe tiff loading
  • [x] Amount of loaded images, images shapes and channels should be correct
  • [x] Precompute average intensity value and hard code in test. Write a test that compares average intensity of loaded images to hardcoded values
• [x] Write segmentation test that checks if masks are non empty
• [x] Write feature extraction tests
• [x] Write test for minmax normalization

We will use the coverage package to monitor code coverage.