update join image function to automatically generate separator images

mu_map/vis/slices.py

-from typing import List
+from typing import List, Optional
 
 import numpy as np
 
 from mu_map.dataset.util import align_images
-from mu_map.file.dicom import load_dcm_img
-from mu_map.file.interfile import load_interfile_img
 from mu_map.file.util import load_img
 
 
 def join_images(
-    images: List[np.ndarray], separator: np.ndarray, vertical: bool = False
-) -> List[np.ndarray]:
+    images: List[np.ndarray],
+    separator: Optional[np.ndarray] = None,
+    vertical: bool = False,
+) -> np.ndarray:
     """
     Create a new image by joining all input images along a separator image.
     If joining horizontally, their shape must be equal along their first axis.
     If joining vertically, their shape must be equal along their second axis.
 
-    :param images: a list of images to join
-    :param separator: a separator image inserted between all images
-    :param vertical: join images vertically instead of horizontally
-    :return: a new image joined as described above
+    Parameters
+    ----------
+    images: list of np.ndarray
+        a list of images to join
+    separator: np.ndarray, optional
+        a separator image inserted between all images - if None a default separator
+        with a width of 10 pixels is created
+    vertical: bool
+        join images vertically instead of horizontally
+
+    Returns
+    -------
+    np.ndarray
+        a new image joined as described above
     """
+    if separator is None:
+        shape = (
+            (10, *images[0].shape[1:])
+            if vertical
+            else (images[0].shape[0], 10, *images[0].shape[2:])
+        )
+        separator = np.full(shape, 239, np.uint8)
+
     res = []
     for image in images:
         res += [image, separator]