Fix access of series to fix unit test

src/pylife/materiallaws/notch_approximation_law.py

@@ -1001,7 +1001,7 @@ def _stress_secondary_branch_multiple_assessment_points_single_lut(self, delta_l
         delta_load = delta_load.fillna(0)
         sign = sign.fillna(0)
 
-        index = self._find_lut_secondary_branch_index_or_raise(delta_load)
+        index = self._find_lut_secondary_branch_index_or_raise(delta_load.values)
 
         return sign.values.flatten() * self._lut_secondary_branch.iloc[(index+1).flatten()].delta_stress.reset_index(drop=True)    # "+1", because the next higher class is used