Name of subsets method is confusing

[Robbybp]

The term "subset" is overloaded in Pyomo right now. I think "subset" should refer the mathematical notion of containment between sets, and the subsets method of Sets, which gives the sets that were used to construct a SetProduct, should be called something else.

Some options:

• coordinate_sets Each coordinate (or tuple of coordinates, for multi-dimensional "coordinate sets") in an element of the SetProduct is an element of some particular set.
• projected_sets These sets contain the projections of every element of the SetProduct onto some coordinate (or tuple of coordinates)
• factor_sets or set_factors Factors are multiplied to form a product. When you "break apart" a product, you get (prime) factors.

I think I favor projected_sets, as coordinate_sets seems to imply that each set occupies a single coordinate, or is 1-dimensional. projected_sets could be confusing as well, as you can project onto any vector (space), but I think it's good enough, and canonically_projected_sets seems like overkill. factor_sets is good too.

[Robbybp]

The name chosen here should probably correspond to the name of the utility function added in #1541