[Cython] Add 'atomic_weight' method and 'atomic_weights' property

Resolves Issue 185.

interfaces/cython/cantera/_cantera.pxd

@@ -69,6 +69,7 @@ cdef extern from "cantera/thermo/ThermoPhase.h" namespace "Cantera":
         size_t nElements()
         size_t elementIndex(string) except +
         string elementName(size_t) except +
+        double atomicWeight(size_t) except +
 
         # species properties
         size_t nSpecies()

interfaces/cython/cantera/thermo.pyx

@@ -175,6 +175,15 @@ cdef class ThermoPhase(_SolutionBase):
         def __get__(self):
             return [self.element_name(m) for m in range(self.n_elements)]
 
+    def atomic_weight(self, m):
+        """Atomic weight [kg/kmol] of element *m*"""
+        return self.thermo.atomicWeight(self.element_index(m))
+
+    property atomic_weights:
+        """Array of atomic weight [kg/kmol] for each element in the mixture."""
+        def __get__(self):
+            return np.array([self.thermo.atomicWeight(m) for m in range(self.n_elements)])
+
     property n_species:
         """Number of species."""
         def __get__(self):