Update Montgomery dataset

Updated the Montgomery dataset and associated tests to fit the new data handling design implemented in the add-datamodule branch

src/ptbench/data/montgomery/default.py

@@ -10,11 +10,11 @@ This configuration:
 
 * Raw data input (on disk):
 
-    * PNG images 12 bit grayscale
+    * PNG images 8 bit grayscale
     * resolution: 4020 x 4892 px or 4892 x 4020 px
 
 * Output image:
-    
+
     * Transforms:
 
         * Load raw PNG with :py:mod:`PIL`
@@ -25,11 +25,11 @@ This configuration:
     * Final specifications
 
         * Fixed resolution: 512 x 512 px
-        * Color RGB encoding
+        * Grayscale (single channel), 8 bits
 
 
 Protocol ``default``:
-    
+
     * Training samples: first 64% of TB and healthy CXR (including labels)
     * Validation samples: 16% of TB and healthy CXR (including labels)
     * Test samples: 20% of TB and healty CXR (including labels)