fix: define ndim in transforms chains

Author: oesteban

nitransforms/manip.py

@@ -67,6 +67,11 @@ def __len__(self):
         """Enable using len()."""
         return len(self.transforms)
 
+    @property
+    def ndim(self):
+        """Get the number of dimensions."""
+        return max(x.ndim for x in self._transforms)
+
     @property
     def transforms(self):
         """Get the internal list of transforms."""