Update src/festim/stepsize.py

Co-authored-by: Rémi Delaporte-Mathurin <[email protected]>

src/festim/stepsize.py

@@ -98,7 +98,7 @@ def max_stepsize(self):
 
     @max_stepsize.setter
     def max_stepsize(self, value):
-        if value is not None and not callable(value):
+        if isinstance(value, float):
             if value < self.initial_value:
                 raise ValueError(
                     "maximum stepsize cannot be less than initial stepsize"