Support JuMP.@objective

This is somewhat related to #4. I'm just creating a separate issue to make tracking a bit easier.

Currently, the use of JuMP.@objective does not seem to be supported. For example, the code

import JuMP
import SNOPT7

m = JuMP.Model(JuMP.optimizer_with_attributes(
    SNOPT7.Optimizer,
    "print_level" => 0,
    "system_information" => "yes",
))

JuMP.@variable(m, x[1:10])
JuMP.@objective(m, Min, x'*x)
JuMP.optimize!(m)

println(JuMP.value.(x))
println(JuMP.termination_status(m))

throws the following error

┌ Error: VariablePrimal not available.
└ @ SNOPT7 ~/.julia/packages/SNOPT7/tZVgh/src/MOIWrapper.jl:420
ERROR: LoadError: type Nothing has no field x
Stacktrace:
 [1] getproperty(::Nothing, ::Symbol) at ./Base.jl:33
 [2] get(::SNOPT7.Optimizer, ::MathOptInterface.VariablePrimal, ::MathOptInterface.VariableIndex) at /home/lassepe/.julia/packages/SNOPT7/tZVgh/src/MOIWrapper.jl:422
 [3] get(::MathOptInterface.Utilities.CachingOptimizer{SNOPT7.Optimizer,MathOptInterface.Utilities.UniversalFallback{MathOptInterface.Utilities.Model{Float64}}}, ::MathOptInterface.VariablePrimal, ::MathOptInterface.VariableIndex) at /home/lassepe/.julia/packages/MathOptInterface/ZJFKw/src/Utilities/cachingoptimizer.jl:605
 [4] get(::MathOptInterface.Bridges.LazyBridgeOptimizer{MathOptInterface.Utilities.CachingOptimizer{SNOPT7.Optimizer,MathOptInterface.Utilities.UniversalFallback{MathOptInterface.Utilities.Model{Float64}}}}, ::MathOptInterface.VariablePrimal, ::MathOptInterface.VariableIndex) at /home/lassepe/.julia/packages/MathOptInterface/ZJFKw/src/Bridges/bridge_optimizer.jl:808
 [5] get(::MathOptInterface.Utilities.CachingOptimizer{MathOptInterface.AbstractOptimizer,MathOptInterface.Utilities.UniversalFallback{MathOptInterface.Utilities.Model{Float64}}}, ::MathOptInterface.VariablePrimal, ::MathOptInterface.VariableIndex) at /home/lassepe/.julia/packages/MathOptInterface/ZJFKw/src/Utilities/cachingoptimizer.jl:605
 [6] _moi_get_result(::MathOptInterface.Utilities.CachingOptimizer{MathOptInterface.AbstractOptimizer,MathOptInterface.Utilities.UniversalFallback{MathOptInterface.Utilities.Model{Float64}}}, ::MathOptInterface.VariablePrimal, ::Vararg{Any,N} where N) at /home/lassepe/.julia/packages/JuMP/y5vgk/src/JuMP.jl:946
 [7] get(::JuMP.Model, ::MathOptInterface.VariablePrimal, ::JuMP.VariableRef) at /home/lassepe/.julia/packages/JuMP/y5vgk/src/JuMP.jl:976
 [8] value(::JuMP.VariableRef; result::Int64) at /home/lassepe/.julia/packages/JuMP/y5vgk/src/variables.jl:766
 [9] value at /home/lassepe/.julia/packages/JuMP/y5vgk/src/variables.jl:766 [inlined]
 [10] _broadcast_getindex_evalf at ./broadcast.jl:648 [inlined]
 [11] _broadcast_getindex at ./broadcast.jl:621 [inlined]
 [12] getindex at ./broadcast.jl:575 [inlined]
 [13] macro expansion at ./broadcast.jl:932 [inlined]
 [14] macro expansion at ./simdloop.jl:77 [inlined]
 [15] copyto! at ./broadcast.jl:931 [inlined]
 [16] copyto! at ./broadcast.jl:886 [inlined]
 [17] copy at ./broadcast.jl:862 [inlined]
 [18] materialize(::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{1},Nothing,typeof(JuMP.value),Tuple{Array{JuMP.VariableRef,1}}}) at ./broadcast.jl:837
 [19] top-level scope at /home/lassepe/worktree/research/20_inverse_games/SNOPT7Lib.jl/example.jl:15
 [20] include(::String) at ./client.jl:457
 [21] top-level scope at REPL[2]:1
in expression starting at /home/lassepe/worktree/research/20_inverse_games/SNOPT7Lib.jl/example.jl:15

snopt / SNOPT7.jl

Support JuMP.@objective #5