[Cython/Examples] Fix extract_submechanism example

The Reaction.listFromFile() method requires a Kinetics instance when
reading a YAML file. In this case, the appropriate Kinetics instance
could be created by Solution('gri30.yaml'). Since we'd have to create
that object anyways to be able to use Reaction.listFromFile(), we might
as well use the Solution.reactions() method, which returns the same
list as Reaction.listFromFile() for this case where the species
definitions are in the same file as the reaction definitions.

Fixes Cantera#821, problem introduced in 1866e35

interfaces/cython/cantera/examples/kinetics/extract_submechanism.py

@@ -14,7 +14,8 @@
 import cantera as ct
 import matplotlib.pyplot as plt
 
-all_species = ct.Species.listFromFile('gri30.yaml')
+input_file = 'gri30.yaml'
+all_species = ct.Species.listFromFile(input_file)
 species = []
 
 # Filter species
@@ -36,7 +37,7 @@
 print('Species: {0}'.format(', '.join(S.name for S in species)))
 
 # Filter reactions, keeping only those that only involve the selected species
-all_reactions = ct.Reaction.listFromFile('gri30.yaml')
+all_reactions = ct.Solution(input_file).reactions()
 reactions = []
 
 print('\nReactions:')
@@ -51,7 +52,7 @@
     print(R.equation)
 print('\n')
 
-gas1 = ct.Solution('gri30.yaml')
+gas1 = ct.Solution(input_file)
 gas2 = ct.Solution(thermo='IdealGas', kinetics='GasKinetics',
                    species=species, reactions=reactions)