Force the use of keywords for optional parameters

Co-authored-by: Johannes Mueller <[email protected]>

src/pylife/materiallaws/notch_approximation_law.py

@@ -272,7 +272,7 @@ def _d_load_secondary_implicit(self, delta_load, delta_stress):
         return -1/delta_stress * self.K_p * self._delta_e_star(delta_load) \
             - delta_load/delta_stress * self.K_p * self._d_delta_e_star(delta_load)
 
-    def stress(self, load, rtol=1e-4, tol=1e-4):
+    def stress(self, load, *, rtol=1e-4, tol=1e-4):
         '''Calculate the stress of the primary path in the stress-strain diagram at a given 
         elastic-plastic stress (load), from a FE computation.
         This is done by solving for the root of f(sigma) in eq. 2.5-45 of FKM nonlinear.