Routines to select and visualize the maxima for a given strict
partial order. This especially includes the computation of the Pareto
frontier, also known as (Top-k) Skyline operator (see Börzsönyi, et al.
Changed object system for preferences from R6 to S4 classes
Moved as.character documentation from general_pref to show.pref
Changed set.df/get.df to assoc.df/assoc.df<-
Improved error messages in psel, get_hasse_diag, ...