Added __ne__ testing

tests/test_model_system.py

@@ -72,11 +72,12 @@ class TestCell:
             ),  # different ordered positions but same cell
         ],
     )
-    def test_eq(self, cell_1: Cell, cell_2: Cell, result: bool):
+    def test_eq_ne(self, cell_1: Cell, cell_2: Cell, result: bool):
         """
-        Test the `__eq__` operator function of `Cell`.
+        Test the `__eq__` and `__ne__` operator functions of `Cell`.
         """
         assert (cell_1 == cell_2) == result
+        assert (cell_1 != cell_2) != result
 
 
 class TestAtomicCell:
@@ -183,11 +184,12 @@ class TestAtomicCell:
             ),  # different ordered positions but same chemical symbols
         ],
     )
-    def test_eq(self, cell_1: Cell, cell_2: Cell, result: bool):
+    def test_eq_ne(self, cell_1: Cell, cell_2: Cell, result: bool):
         """
-        Test the `__eq__` operator function of `AtomicCell`.
+        Test the `__eq__` and `__ne__` operator functions of `AtomicCell`.
         """
         assert (cell_1 == cell_2) == result
+        assert (cell_1 != cell_2) != result
 
     @pytest.mark.parametrize(
         'chemical_symbols, atomic_numbers, formula, lattice_vectors, positions, periodic_boundary_conditions',