WIP: Support absolute goal order for minimization goals

One would typically use order=2 to minimize the absolute value of the goal function, but that makes nominals a bit harder and easily leads to scaling issues. A more involved option that was used is to introduce an auxiliary variable and additional constraints to minimize the absolute value of the goal function.

It is the latter logic that this commit adds to GoalProgrammingMixin, such that the user now only has to set order="abs" on a Goal instance. The auxiliary variable and accompanying constraints are automatically added behind the scenes.

Closes #1060 (closed)

TODO:

• Do not abuse goal order for this, but introduce a new flag.