add rigorous option for HXutilities

Author: yoelcortes

biosteam/units/heat_exchange.py

@@ -741,15 +741,15 @@ def _design(self):
                     hx = self.auxiliary(
                         'heat_exchangers', bst.HXutility,
                         ins=intermediate, T=T_intermediate,
-                        rigorous=True
+                        rigorous=self.rigorous,
                     )
                     intermediate = hx.outs[0]
                     hx.simulate()
                 elif T_out <= T_in if heating else T_out > T_in:
                     hx = self.auxiliary(
                         'heat_exchangers', bst.HXutility,
                         ins=intermediate, outs=product, T=T_out,
-                        rigorous=True
+                        rigorous=self.rigorous,
                     )
                     hx.simulate()
                     break