Fixed _ImageClassifier._label_unclassified_pixels() overwriting nodata values.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>

gms_preprocessing/algorithms/classification.py

@@ -114,7 +114,11 @@ class _ImageClassifier(object):
         else:
             raise ValueError(threshold)
 
-        cmap[distances > threshold] = label_unclassified
+        mask_unclassified = distances > threshold
+        if cmap_nodataVal is not None:
+            mask_unclassified = mask_unclassified & (cmap[:] != cmap_nodataVal)
+
+        cmap[mask_unclassified] = label_unclassified
         cmap.unclassified_val = label_unclassified
 
         return cmap