Turning on compression for SimpleITK writer.

When saving segmentation in nrrd format without compression the file
size is significantly larger than with compression, 20mb versus 19kb
for a 5000x4000 nrrd binary segmentation.

nnunetv2/imageio/simpleitk_reader_writer.py

@@ -126,4 +126,4 @@ def write_seg(self, seg: np.ndarray, output_fname: str, properties: dict) -> Non
         itk_image.SetOrigin(properties['sitk_stuff']['origin'])
         itk_image.SetDirection(properties['sitk_stuff']['direction'])
 
-        sitk.WriteImage(itk_image, output_fname)
+        sitk.WriteImage(itk_image, output_fname, True)