First version of dataset class

I have made a first version of the dataset class for loading images and target segmentations. Also changed 'tools' folder to 'utils' which have caused minor changes in import statements in several other files. Also made a notebook with the U-Net pipeline with the different tools used in action.

qim3d/utils/data.py

@@ -49,9 +49,9 @@ class Dataset(torch.utils.data.Dataset):
 
         self.sample_images = [file for file in sorted((path / "images").iterdir())]
         self.sample_targets = [file for file in sorted((path / "labels").iterdir())]
-
-    def __len__(self):
         assert len(self.sample_images) == len(self.sample_targets)
+        
+    def __len__(self):
         return len(self.sample_images)
 
     def __getitem__(self, idx):