add optional weights to intersection area

Allow users to specify hints to how important each intersection area is when
laying out venn diagrams.

Fairly niche use case - but might help fix issue #9
Merge branch 'master' into add_weights
fix greedyLayout with weighted input

There was a bug in applying weights for non-zero intersection sizes. The
greedy layout call wasn't considering the weights, leading to generating the
same initial layout as in the unweighted version. This frequently lead to
layouts that the optimizer couldn't step out of.

Fix this by considering weights when determining which set to lay out first in
the greedyLayout call
Merge pull request #10 from benfred/add_weights

add optional weights to intersection area