fix: consistently choose the reference with new convention

unit tests are your friend

Resolves: #81.

Author: oesteban

nitransforms/manip.py

@@ -74,8 +74,8 @@ def transforms(self):
     @transforms.setter
     def transforms(self, value):
         self._transforms = _as_chain(value)
-        if self.transforms[-1].reference:
-            self.reference = self.transforms[-1].reference
+        if self.transforms[0].reference:
+            self.reference = self.transforms[0].reference
 
     def append(self, x):
         """