Add get_variables dispatch for Reaction

[TorkelE]

The get_variables function is currently defined for Equations but not Reactions. I've added it. It goes through all fields, checking and adds all symbolic variables to an output vector.

[isaacsas]

If we are going to add it we should probably add the in-place version and then just call that for the allocating version. It might also be worth using a Set and converting to a vector at the end instead. See https://github.com/JuliaSymbolics/Symbolics.jl/blob/1da13fdc375fccd6cd0a779a53b06be4ec7a54ec/src/utils.jl#L47-L79

[isaacsas]

Actually, we already have an inplace version that takes Sets, so why not just wrap that and then convert to a vector at the end?

[TorkelE]

In practise, I just used what Symbolics define for Equations:

get_variables(eq::Equation) = unique(vcat(get_variables(eq.lhs), get_variables(eq.rhs)))

and I figured the above definition was most faithful to that.

Little bit uncertain exactly what rewrite you want, but if you have a different way to phrase the function content I am happy to update.

[isaacsas]

A few minor changes. Once addressed and tests pass feel free to merge.

[isaacsas]

Shouldn't this check if stoich is a Symbolic? If not there is no reason to call get_variables!?

[TorkelE]

Yeah, that is definitely an improvement.