Fix "colour.TriSpectralPowerDistribution.items" method.

Closes #331.

colour/colorimetry/spectrum.py

@@ -2682,13 +2682,21 @@ def items(self):
         Property for **self.items** attribute. This is a convenient attribute
         used to iterate over the tri-spectral power distribution.
 
+        Notes
+        -----
+        -   In contrast to :func:`SpectralPowerDistribution.items` method,
+            :func:`TriSpectralPowerDistribution.items` returns a `list` as it
+            is not possible to convert its output to a `ndarray`. It is
+            possible to separate wavelengths from data by unpacking the return
+            value into the `zip` definition as follows: `zip(*tri_spd.items)`.
+
         Returns
         -------
-        ndarray
+        list
             Tri-spectral power distribution data.
         """
 
-        return np.array(list(self.__iter__()))
+        return list(self.__iter__())
 
     @items.setter
     def items(self, value):

colour/colorimetry/tests/tests_spectrum.py

@@ -2143,6 +2143,16 @@ def test_values(self):
             self._spd.values,
             [v for k, v in sorted(SAMPLE_SPD_DATA.items())])
 
+    def test_items(self):
+        """
+        Tests :attr:`colour.colorimetry.spectrum.\
+SpectralPowerDistribution.items` attribute.
+        """
+
+        np.testing.assert_array_equal(
+            self._spd.items,
+            tstack((self._spd.wavelengths, self._spd.values)))
+
     def test_shape(self):
         """
         Tests :attr:`colour.colorimetry.spectrum.\
@@ -2598,6 +2608,20 @@ def test_values(self):
                 [v for k, v in sorted(
                     CIE_1931_2_DEGREE_STANDARD_OBSERVER['z_bar'].items())])))
 
+    def test_items(self):
+        """
+        Tests :attr:`colour.colorimetry.spectrum.\
+TriSpectralPowerDistribution.items` attribute.
+        """
+
+        np.testing.assert_array_equal(
+            list(zip(*self._tri_spd.items))[0],
+            self._tri_spd.wavelengths)
+
+        np.testing.assert_array_equal(
+            list(zip(*self._tri_spd.items))[1],
+            self._tri_spd.values)
+
     def test_shape(self):
         """
         Tests :attr:`colour.colorimetry.spectrum.\