Implement multinet

[bmihaljevic]

Allow for maximizing BIC/AIC and not just loglik. Perhaps combine with arbitrary structure search algorithms for each net.

[bmihaljevic]

We need an 'bnc_ensemble' class to generalize both AODE and multinet. Need to update the models() function. Implement and test both complete and incomplete prediction with multinet. Then add it to the documentation.