bs_predict_vote: improved strict majority handling + added support of relative majority

Following my exchange with Ariel and Zhen for bs_predict_vote:
- slight modification of BM algorithm.
- added a bloc in BM to check if a second process is needed (check if distinct votes are more than 2 => majority result must be check and we will use relative majority otherwise).
- support of multiclass case and relative majority.
- should work with negatives.
- votes rounded to int => according to Zhen: bs_vote is for classification, this rounding improves accuracy. This part could be enriched using link().
- tested speed comparison on large dataset and compared with bs_mean: same performance than before, time nearly equal between mean and vote. 

PS: adding a new test for multiclass case would be good but lack of time.