Fix to Triangulation -> Link -> Triangulation reversing orientation

If a Triangulation is specified by a DT code *with flips*, then one
could *sometimes* change the orientation of the exterior by what should
have been a no-op::

   >>> E0 = snappy.Manifold('DT[eaebdeac.11100]')
   >>> E0.chern_simons()
   0.153204133297152
   >>> E0.link().exterior().chern_simons()
   -0.153204133297152

This only happened in certain circumstances, and the DT codes in
HTLinkExteriors were seemingly immune.

This bug had the same root cause as dd9a2e7.

cython/core/triangulation.pyx

@@ -352,7 +352,7 @@ cdef class Triangulation():
         if self._PDcode is not None:
             return spherogram.Link(self._PDcode)
         elif self.DT_code() is not None:
-            return spherogram.DTcodec(self.DT_code()).link()
+            return spherogram.DTcodec(*self.DT_code(flips=True)).link()
         else:
             raise ValueError('No associated link known.')