User must be able to specify different best subsets criteria and return a union or intersection of all the subsets.

[linusjf]

Brief description of the problem:
Best subsets criteria.

The user must be able to specify different criteria such as AIC, SBC etc and specify whether he or she needs a union or intersection of the results.

The union returns all the models that meet the criteria individually i.e., it is a union of the subsets that are returned.
The intersection returns the models that meet all the criteria i.e., all the models meet every one of the criteria, a set intersection of all the subsets that are returned.

#157 (comment)