TEST: Test Image.nvals property

tests/test_image.py

@@ -306,6 +306,7 @@ def _test_Image_atts(imgtype):
             assert tuple(i.nibImage.shape)              == tuple(dims)
             assert tuple(i.nibImage.header.get_zooms()) == tuple(pixdims)
 
+            asert  i.nvals      == 1
             assert i.ndim       == expndims
             assert i.dtype      == dtype
             assert i.name       == op.basename(path)
@@ -1170,4 +1171,5 @@ def test_rgb_image():
 
         img = fslimage.Image('rgb.nii')
 
+        assert img.nvals     == 3
         assert img.dataRange == (0, 255)