[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:

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