Instance checker in void mat (GEOUNED-org#267)

* Update data_classes.py

fix bad black format of PR GEOUNED-org#265 and add small modifications

* Update data_classes.py

black is black

src/geouned/GEOUNED/utils/data_classes.py

@@ -769,17 +769,18 @@ def voidMat(self):
     def voidMat(self, voidMat: list):
         if not isinstance(voidMat, list):
             raise TypeError(f"geouned.Settings.voidMat should be a list, not a {type(voidMat)}")
-        if len(voidMat) == 0:
-            self._voidMat = voidMat
-        else:
-            if not isinstance(voidMat[0], int):
-                raise TypeError(f"first entry of geouned.Settings.voidMat should be an int, not a {type(entry)}")
-            if not isinstance(voidMat[1], int):
-                if not isinstance(voidMat[1], float):
-                    raise TypeError(f"second entry of geouned.Settings.voidMat should be an int or float, not a {type(entry)}")
-            if not isinstance(voidMat[2], str):
-                raise TypeError(f"third entry of geouned.Settings.voidMat should be a str, not a {type(entry)}")
-            self._voidMat = voidMat
+        if len(voidMat) == 3:
+            entry0, entry1, entry2 = voidMat
+            if not isinstance(entry0, int):
+                raise TypeError(f"first entry of geouned.Settings.voidMat should be an int, not a {type(entry0)}")
+            if not isinstance(entry1, int):
+                if not isinstance(entry1, float):
+                    raise TypeError(f"second entry of geouned.Settings.voidMat should be an int or float, not a {type(entry1)}")
+            if not isinstance(entry2, str):
+                raise TypeError(f"third entry of geouned.Settings.voidMat should be a str, not a {type(entry2)}")
+        elif len(voidMat) > 0:
+            raise TypeError(f"geouned.Settings.voidMat should be a list with 3 elements or void list")
+        self._voidMat = voidMat
 
     @property
     def voidExclude(self):