add n_intersections to jaccard output

jaccard is very useful to gauge the amount of overlap. this pull request adds n_intersections to the outputs that shows the number of unique intersections in -a that overlap with -b.

between the bases of overlap (intersection)

the proportion of overlap (jaccard)

and now the number of intersections (n_intersections)

this gives the most commonly used (simple) metrics when comparing 2 datasets.