consistent arg order for `fix_mass_balanced`

...together with the other commits in this branch, this:

Closes #330

docs/src/notebooks/3_basic_stdmodel_usage.jl

@@ -175,8 +175,8 @@ check_duplicate_reaction(pgm_duplicate, model.reactions; only_metabolites = fals
 
 # ## Checking the internals of `StandardModel`s: `is_mass_balanced`
 
-# Finally, `is_mass_balanced` can be used to check if a reaction is mass
+# Finally, [`is_mass_balanced`](@ref) can be used to check if a reaction is mass
 # balanced based on the formulas of the reaction equation.
 
 pgm_duplicate.metabolites = Dict{String,Float64}("3pg_c" => 1, "2pg_c" => -1, "h2o_c" => 1) # not mass balanced now
-is_bal, extra_atoms = is_mass_balanced(pgm_duplicate, model) # extra_atoms shows which atoms are in excess/deficit
+is_bal, extra_atoms = is_mass_balanced(model, pgm_duplicate) # extra_atoms shows which atoms are in excess/deficit

src/base/utils/Reaction.jl

@@ -50,14 +50,14 @@ function is_boundary(rxn::Reaction)::Bool
 end
 
 """
-    is_mass_balanced(rxn::Reaction, model::StandardModel)
+    is_mass_balanced(model::StandardModel, rxn::Reaction)
 
 Checks if `rxn` is atom balanced. Returns a boolean for whether the reaction is balanced,
 and the associated balance of atoms for convenience (useful if not balanced).
 
 See also: [`get_atoms`](@ref), [`check_duplicate_reaction`](@ref)
 """
-function is_mass_balanced(rxn::Reaction, model::StandardModel)
+function is_mass_balanced(model::StandardModel, rxn::Reaction)
     atom_balances = Dict{String,Float64}() # float here because stoichiometry is not Int
     for (met, stoich) in rxn.metabolites
         atoms = metabolite_formula(model, met)