[Python] Add Reaction.__contains__ method

interfaces/cython/cantera/reaction.pyx

@@ -161,6 +161,9 @@ cdef class Reaction:
         def __set__(self, products):
             self.reaction.products = comp_map(products)
 
+    def __contains__(self, species):
+        return species in self.reactants or species in self.products
+
     property orders:
         """
         Get/Set the reaction order with respect to specific species as a dict

interfaces/cython/cantera/test/test_kinetics.py

@@ -613,6 +613,9 @@ class TestReaction(utilities.CanteraTest):
         self.assertEqual(r.products['O2'], 1)
         self.assertEqual(r.efficiencies['H2O'], 15.4)
         self.assertEqual(r.rate.temperature_exponent, -1.0)
+        self.assertIn('O', r)
+        self.assertIn('O2', r)
+        self.assertNotIn('H2O', r)
 
     def test_fromXml(self):
         import xml.etree.ElementTree as ET