clean up some leftover unions with Nothing

src/analysis/flux_balance_analysis.jl

 """
-    flux_balance_analysis_vec(args...)::Union{Vector{Float64},Nothing}
+    flux_balance_analysis_vec(args...)::Maybe{Vector{Float64}}
 
 A variant of FBA that returns a vector of fluxes in the same order as reactions
 of the model, if the solution is found.
@@ -13,7 +13,7 @@ flux_balance_analysis_vec(args...; kwargs...)::Maybe{Vector{Float64}} =
     flux_vector(flux_balance_analysis(args...; kwargs...))
 
 """
-    flux_balance_analysis_dict(model::MetabolicModel, args...)::Union{Dict{String, Float64},Nothing}
+    flux_balance_analysis_dict(model::MetabolicModel, args...)::Maybe{Dict{String, Float64}}
 
 A variant of FBA that returns a dictionary assigning fluxes to reactions, if
 the solution is found. Arguments are passed to [`flux_balance_analysis`](@ref).

src/analysis/screening.jl

@@ -175,7 +175,7 @@ screen_variants(model, variants, analysis; workers = [myid()]) =
     screen(model; variants = variants, analysis = analysis, workers = workers)
 
 """
-    screen_optimize_objective(_, optmodel)::Union{Float64,Nothing}
+    screen_optimize_objective(_, optmodel)::Maybe{Float64}
 
 A variant of [`optimize_objective`](@ref) directly usable in
 [`screen_optmodel_modifications`](@ref).

src/base/types/FluxVariabilitySummary.jl

@@ -4,8 +4,8 @@
 Stores summary information about the result of a flux variability analysis.
 """
 struct FluxVariabilitySummary
-    biomass_fluxes::Dict{String,Vector{Union{Float64,Nothing}}}
-    exchange_fluxes::Dict{String,Vector{Union{Float64,Nothing}}}
+    biomass_fluxes::Dict{String,Vector{Maybe{Float64}}}
+    exchange_fluxes::Dict{String,Vector{Maybe{Float64}}}
 end
 
 """
@@ -15,8 +15,8 @@ A default empty constructor for [`FluxVariabilitySummary`](@ref).
 """
 function FluxVariabilitySummary()
     FluxVariabilitySummary(
-        Dict{String,Vector{Union{Float64,Nothing}}}(),
-        Dict{String,Vector{Union{Float64,Nothing}}}(),
+        Dict{String,Vector{Maybe{Float64}}}(),
+        Dict{String,Vector{Maybe{Float64}}}(),
     )
 end
 
@@ -84,14 +84,14 @@ function flux_variability_summary(
         rxn_ids,
     )
 
-    biomass_fluxes = Dict{String,Vector{Union{Float64,Nothing}}}()
+    biomass_fluxes = Dict{String,Vector{Maybe{Float64}}}()
     for rxn_id in bmasses
         lb = isnothing(flux_result[1][rxn_id]) ? nothing : flux_result[1][rxn_id][rxn_id]
         ub = isnothing(flux_result[2][rxn_id]) ? nothing : flux_result[2][rxn_id][rxn_id]
         biomass_fluxes[rxn_id] = [lb, ub]
     end
 
-    ex_rxn_fluxes = Dict{String,Vector{Union{Float64,Nothing}}}()
+    ex_rxn_fluxes = Dict{String,Vector{Maybe{Float64}}}()
     for rxn_id in ex_rxns
         lb = isnothing(flux_result[1][rxn_id]) ? nothing : flux_result[1][rxn_id][rxn_id]
         ub = isnothing(flux_result[2][rxn_id]) ? nothing : flux_result[2][rxn_id][rxn_id]