The LinearConstraintSet shall return its constraints in a deterministic way

As previously discussed on the mailinglist, the LinearConstraintSet should return its internally stored LinearConstraints in the same iteration order as they have been provided via its constructor.

This ensures that the execution of the same linear problem results in the same results each time it is executed. This is especially important when linear problems are loaded from a file, e.g. mps format, and makes it simpler to debug problems and compare with other solvers which do the same thing.