Sorting mixed-type Sets fails in Python3

[jsiirola]

PR #305 pointed out an issue with Sets in Python 3: while Pyomo allows mixed-type Sets (i.e., mixing integers and strings in a single set), doing so will likely fail when running under Python 3 due to pervasive sorting to enforce determinism.

We should propagate the more robust sorter added in #305 to the main Set class as a sorted_values() method, and then replace most uses of sorted() with set.sorted_values() throughout the code.

[carldlaird]

At this same time, I think we should also consider (1) defaulting all sets to ordered, or (2) removing unsorted sets altogether.

[jsiirola]

Agreed. I would like to have a general design discussion as part of #326, and defaulting Sets to ordered is definitely part of that.