[Python] Allow setting composition of sliced Solution objects

The following syntax:

    >>> phase['H2','O2'].X = [1.0, 0.5]

Is equivalent to:

    >>> phase.X = 'H2:1.0, O2:0.5'

interfaces/cython/cantera/test/test_thermo.py

@@ -104,6 +104,16 @@ class TestThermoPhase(utilities.CanteraTest):
         with self.assertRaises(Exception):
             self.phase.Y = {'H2':1.0, 'O2':'xx'}
 
+    def test_setCompositionSlice(self):
+        self.phase['H2', 'O2'].X = 0.1, 0.9
+        X = self.phase.X
+        self.assertNear(X[0], 0.1)
+        self.assertNear(X[3], 0.9)
+
+    def test_setCompositionSlice_bad(self):
+        with self.assertRaises(ValueError):
+            self.phase['H2','O2'].Y = [0.1, 0.2, 0.3]
+
     def test_report(self):
         report = self.phase.report()
         self.assertTrue(self.phase.name in report)

interfaces/cython/cantera/thermo.pyx

@@ -246,11 +246,16 @@ cdef class ThermoPhase(_SolutionBase):
             return data
 
     cdef void _setArray1(self, thermoMethod1d method, values) except *:
-        if len(values) != self.n_species:
-            raise ValueError("Array has incorrect length")
+        cdef np.ndarray[np.double_t, ndim=1] data
 
-        cdef np.ndarray[np.double_t, ndim=1] data = \
-            np.ascontiguousarray(values, dtype=np.double)
+        if len(values) == self.n_species:
+            data = np.ascontiguousarray(values, dtype=np.double)
+        elif len(values) == len(self._selected_species):
+            data = np.zeros(self.n_species, dtype=np.double)
+            for i,k in enumerate(self._selected_species):
+                data[k] = values[i]
+        else:
+            raise ValueError("Array has incorrect length")
         method(self.thermo, &data[0])
 
     property molecular_weights: