fix attribute lookup in sep_rho

Author: bknueven

Diff for: mpisppy/extensions/sep_rho.py

@@ -141,7 +141,7 @@ def _compute_xmin(ph):
     def nonant_cost_coeffs(self, s):
         if s not in self._nonant_cost_coeffs:
             if hasattr(s._mpisppy_data, "nonant_cost_coeffs"):
-                self._nonant_cost_coeffs[s] = s._mpisppy_model.nonant_cost_coeffs
+                self._nonant_cost_coeffs[s] = s._mpisppy_data.nonant_cost_coeffs
             else:
                 self._nonant_cost_coeffs[s] = nonant_cost_coeffs(s)
         return self._nonant_cost_coeffs[s]