Average shared span for multiple matches

[hfr1tz3]

Option 2 for issue #11

For each node $n_2\in T_2$ with multiple matches in $T_1$, Let $\beta(n_2)=[n_1\in T_1 \colon \alpha(n_1)=n_2]$. Then we compute the similarity between two trees $T_1$, and $T_2$ as
$$sim(T_1,T_2)=\sum_{n_2\in T_2}\frac{1}{\beta(n_2)}\sum_{n_1\in T_1} m(n_1,n_2),$$
which is the average over shared spans between nodes in $T_2$ and their multiple matches $\beta(n_2)$.

[hfr1tz3]

I added extra tests that are for the ties='average' method. I am open to more suggestions if we need more.