Add a possibility to allow the universal empty set

[AlexisDerumigny]

Thanks for this package, it is very helpful!

In my application, it is also of interest to allow the universal empty set (i.e. if some rows of the dataset have no answer selected).

This PR adds a new option keep.universal.empty.set which is FALSE by default (to be back-compatible). When keep.universal.empty.set = TRUE, the output graph also includes the universal empty set as well as all the other sets that have been chosen, as usual.