fix: ensure __len__ is defined for all transforms``

nitransforms/base.py

@@ -279,6 +279,16 @@ def __add__(self, b):
 
         return TransformChain(transforms=[self, b])
 
+    def __len__(self):
+        """
+        Enable ``len()``.
+        
+        By default, all transforms are of length one.
+        This must be overriden by transforms arrays and chains.
+
+        """
+        return 1
+
     @property
     def reference(self):
         """Access a reference space where data will be resampled onto."""

nitransforms/nonlinear.py

@@ -94,10 +94,6 @@ def __repr__(self):
         """Beautify the python representation."""
         return f"<{self.__class__.__name__}[{self._field.shape[-1]}D] {self._field.shape[:3]}>"
 
-    def __len__(self):
-        """Enable len() -- for compatibility, only len == 1 is supported."""
-        return 1
-
     @property
     def ndim(self):
         """Get the dimensions of the transform."""