minor changes for future work

Model.py

@@ -348,8 +348,8 @@ class Model:
             FCa_0,  # FCa
         ]
 
-    def calculated_current():
-        return None
+    def calculated_current(self):
+        return 
 
     def solve(self, initial_values, tspan, dt, times):
         times = np.arange(*tspan, dt)