Fix discriminating_bscore::best_possible_bscore()

I'm sure the fix is correct for prerec_bscore (I tested it, though no
utest, sorry, no time).

I believe it is correct for recall_bscore but I didn't test it.

I am almost sure it is not correct for bep_bscore and f_one_bscore.
Add todo item about discriminating_bscore::best_possible_bscore()