Make mix_materials() inherit "depletable" attribute of involved materials

openmc/material.py

@@ -1079,6 +1079,13 @@ class Material(IDManagerMixin):
         new_density = np.sum([dens for dens in mass_per_cc.values()])
         new_mat.set_density('g/cm3', new_density)
 
+        # If any of the involved materials is depletable, the new material is 
+        # depletable
+        for mat in materials:
+            if mat.depletable:
+                new_mat.depletable = True
+                break
+
         return new_mat
 
     @classmethod