Ensure the same four component materials form the same ID.

gprMax/cython/yee_cell_build.pyx

@@ -53,17 +53,22 @@ cpdef void create_electric_average(
         G: FDTDGrid class describing a grid in a model.
     """
 
-    # Make an ID composed of the names of the four materials that will be averaged
-    requiredID = (G.materials[numID1].ID + '+' + G.materials[numID2].ID + '+' +
-                  G.materials[numID3].ID + '+' + G.materials[numID4].ID)
+    # Make an ID composed of the names of the four materials that will
+    # be averaged. Sort the names to ensure the same four component
+    # materials always form the same ID.
+    requiredID = "+".join(
+        sorted(
+            [
+                G.materials[numID1].ID,
+                G.materials[numID2].ID,
+                G.materials[numID3].ID,
+                G.materials[numID4].ID,
+            ]
+        )
+    )
 
     # Check if this material already exists
-    tmp = requiredID.split('+')
-    material = [x for x in G.materials if
-             x.ID.count(tmp[0]) == requiredID.count(tmp[0]) and
-             x.ID.count(tmp[1]) == requiredID.count(tmp[1]) and
-             x.ID.count(tmp[2]) == requiredID.count(tmp[2]) and
-             x.ID.count(tmp[3]) == requiredID.count(tmp[3])]
+    material = [x for x in G.materials if x.ID == requiredID]
 
     if material:
         G.ID[componentID, i, j, k] = material[0].numID