fix: squeeze last dimension of moving image in apply()

Resolves: #270.

Author: oesteban

nitransforms/resampling.py

@@ -19,6 +19,7 @@
 from nibabel.loadsave import load as _nbload
 from nibabel.arrayproxy import get_obj_dtype
 from nibabel.spatialimages import SpatialImage
+from nibabel.funcs import squeeze_image
 from scipy import ndimage as ndi
 
 from nitransforms.base import (
@@ -233,6 +234,8 @@ def apply(
     if isinstance(spatialimage, (str, Path)):
         spatialimage = _nbload(str(spatialimage))
 
+    spatialimage = squeeze_image(spatialimage)
+
     # Avoid opening the data array just yet
     input_dtype = cap_dtype(get_obj_dtype(spatialimage.dataobj), dtype_width)