Many improvements for sets-to/from-pairs and equivalence.

* [ASSOC_EXT,SC] Variable renaming.
* [EQUIV,NEW] New module supporting equivalence relations. Currently only including predicate number_of_equivalence_pairs/3.
* [PAIR_EXT,EXT] set_to_pairs/3 now takes a comparator, allowing symmetric and reflexive pairs to be excluded.
* [PAIR_EXT,EXT] sets_to_pairs/3 now take options that are translated into a comparator, allowing symmetric and reflexive pairs to be excluded.
* [PAIR_EXT,SC] Simplified pairs_to_sets/3 by using select/3 i.o. member/2.
* [PAIR_EXT,FIX] read_pairs_from_file/2 returns an ordered set of pairs now.
* [PAIR_EXT,EQUIV,SC] Merge modules.
* [PAIR_EXT,SC] Relocated reflexive_pair/1.
* [PDF,FIX] Missing module import.