ChemicalSpecies and Symbol == only works one way

The example speaks for itself:
```julia-repl
julia> ChemicalSpecies(:Si) == :Si
true

julia> :Si == ChemicalSpecies(:Si)
false
```

Looks like while `==(a::ChemicalSpecies, sym::Symbol)` was implemented, a corresponding `==(sym::Symbol, a::ChemicalSpecies)` is missing.