Implement TREE frequency weight system

[tngreene]

Right now we're using a frequency prop which you'd still have to do the math to make sure that each TREE with that layer number adds up to 1.0. Petr and I had this exchange:

I'd like to know a little more about what you meant by "weighted importance".

If you have Trees A, B, C, D and they should ultimately have the frequencies (not that you would normally hand calculate this)

[.5, .25, .125, .125]
What values would you put in the UI for each tree?

tree_freq = (tree_weight / sum_of_all_weights) * 100

So for your example I can use weights like:
2, 1, 0.5, 0.5
or
4, 2, 1, 1

Seems fine enough! Though I'd like to stick to ints.

• Replace frequency prop with Weighted Importance prop
• Unit test showing that weights are properly used and all frequencies - per layer number (we'll have multiple layer numbers) add up to 100
• Code for importance feature

[tngreene]

While fixing this, I also made some changes to is_vertical that I don't think were right. We'll see!