[ExaModelsJuMP] Wrapper is broken if JuMP's objective sense is set to Max

[frapac]

Please find attached a MWE:

using Test
using JuMP
using ExaModels
using MadNLP

model = Model()
@variable(model, 0 <= x <= 1)
@objective(model, Max, x^2)

# Optimize with JuMP
JuMP.set_optimizer(model, MadNLP.Optimizer)
JuMP.optimize!(model)
@test JuMP.objective_value(model) ≈ 1.0 rtol=1e-7

# Optimize with ExaModels
exam = ExaModels.ExaModel(model)
results = madnlp(exam)
@test results.objective ≈ 1.0 rtol=1e-7 #broken!

[sshin23]

Addressed in #66