[WIP] Improve barrier reformulation

[cvanaret]

Interior-point methods: handle the slack variables in PrimalDualInteriorPointProblem instead of reformulating the Model into a HomogeneousEqualityConstrainedModel. Several goals:

• handle more structure (e.g. the slacks produce a positive definite block in the Hessian that need not be regularized, the inequality constraints get full row rank) at the subproblem level.
• switch between infeasible (with slacks) and feasible (without slacks) methods more easily.

Milestones:

• [x] move HomogeneousEqualityConstrainedModel into PrimalDualInteriorPointProblem
• [ ] move FixedBoundsConstrainedModel into PrimalDualInteriorPointProblem
• [ ] delete ScaledModel and create an optional Scaling object in Uno
• [x] remove ModelFactory
• [x] add write_solution in AMPLModel
• [x] create reformulations on the fly instead of storing them
• [ ] fix computation of infeasibility measure
• [x] fix computation of least-square multipliers