fix: indeed, asaffine() had the reversed convention

Thanks Chris for pointing this out.

Co-authored-by: Chris Markiewicz <[email protected]>

nitransforms/manip.py

@@ -156,15 +156,25 @@ def asaffine(self, indices=None):
                [0., 0., 1., 0.],
                [0., 0., 0., 1.]])
 
+        >>> chain = TransformChain(transforms=[
+        ...     Affine.from_matvec(vec=(1, 2, 3)),
+        ...     Affine.from_matvec(mat=[[0, 1, 0], [0, 0, 1], [1, 0, 0]]),
+        ... ])
+        >>> chain.asaffine()
+        array([[0., 1., 0., 2.],
+               [0., 0., 1., 3.],
+               [1., 0., 0., 1.],
+               [0., 0., 0., 1.]])
+
         Parameters
         ----------
         indices : :obj:`numpy.array_like`
             The indices of the values to extract.
 
         """
         affines = self.transforms if indices is None else np.take(self.transforms, indices)
-        retval = affines[0]
-        for xfm in affines[1:]:
+        retval = affines[-1]
+        for xfm in reversed(affines[:-1]):
             retval @= xfm
         return retval