added setter and getter for decay constant

festim/sources/radioactive_decay.py

@@ -6,5 +6,17 @@ def __init__(self, decay_constant, volume, field="all") -> None:
         self.decay_constant = decay_constant
         super().__init__(value=None, volume=volume, field=field)
 
+    @property
+    def decay_constant(self):
+        return self._decay_constant
+
+    @decay_constant.setter
+    def decay_constant(self, value):
+        if not isinstance(value, (float, int)):
+            raise TypeError("decay_constant must be a float or an int")
+        if value <= 0:
+            raise ValueError("decay_constant must be positive")
+        self._decay_constant = value
+
     def form(self, concentration):
         return self.decay_constant * concentration