Saving plsyn to file instead of stdio

[RdR1024]

The current git download of the thea libraries, the save_axioms hook in the owl2_plsyn.pl file doesn’t save to file, but instead dumps to screen.

The following fix in the predicate owl2_io:save_axioms_hook/3 will alleviate that. I essentially mirrored the code from the similar file owl2_to_prolog_dlp.pl.

owl2_io:save_axioms_hook(File,plsyn,Opts) :-
( nonvar(File)
-> tell(File)
; true
),
write_owl_as_plsyn(Opts),
told.