Ordering of species in reaction is nondeterministic

[connie]

When a reaction string is printed in Chemkin in different jobs, the ordering in which they are listed gets jumbled. For instance in 2 jobs, I can get:

CH3(17)+CH3(17)=H(2)+C2H5(27)                       7.845e+12 0.100     10.600   

and

CH3(17)+CH3(17)=C2H5(27)+H(2)                       7.845e+12 0.100     10.600  

Which are identical except for the ordering. This is correct but makes it difficult for the user when one has to search multiple permutations of a single reaction to identify it. I think we should try to make str(reaction) print the species in order of index.

[rwest]

#388 may be relevant here

[mliu49]

Species sorting now uses comparison methods (implemented in 0c0c7ed), which use the new sorting_key property (implemented in c90c407). This will provide deterministic sorting, although the current implementation depends on the species label and index, which can change depending on thermo libraries and the order that species are generated.

In cases like this, where the indices are the same, this will guarantee deterministic results.