Allow "absolute" goal order for minimization goals
Currently one would use order=2 to minimize the absolute value, but that makes nominals a bit harder (and can lead to other scaling issues). As it's a common wish, it would be nice to allow order to be e.g. the string "abs" as well for minimization goals (or maybe a separate flag?). The boilerplate of adding auxiliary variables and addiing the + and - constraints can then be handled behind the scenes by goal programming mixin.