Canonicalization of reactions and multi-SMILES: component-wise, or global?

[avaucher]

The current implementations of canonicalize_multicomponent_smiles and canonicalize_compounds, are doing molecule-wise canonicalizations, not changing the order of molecules.

As raised in a discussion with @A-Thakkar, one may also expect the canonicalization to take care of reordering the components: one may expect to obtain the same representation when "canonicalizing" two identical reactions where only the order of the compounds is different.

It is still unclear which of the two approaches should be preferred.