Fixed unwanted copy of DualResiduals

[cvanaret]

Replaced

const auto residuals = this->solving_feasibility_problem ? current_iterate.feasibility_residuals : current_iterate.residuals;

with

const auto& residuals = this->solving_feasibility_problem ? current_iterate.feasibility_residuals : current_iterate.residuals;

in PrimalDualInteriorPointSubproblem, thus getting rid of an unwanted copy.