explore scaling of extension tree branch lengths relative to foundation tree

[gregcaporaso]

One thing we could try here would be to add a multiplicative factor to the cli that gives users the ability to multiply branches in the extension tree by that value (though I realize that might be overly simplistic, and I'm open to other suggestions). This value would need to always be greater than zero, and a value of 1 would correspond to no scaling. For the paper, we can then test varying this parameter to determine if it affects downstream diversity metrics in a meaningful way (if the resulting UniFrac distance matrices are highly correlated by Mantel r, then it doesn't), and if so experiment with to optimize our detection of the small and large effect sizes. Thoughts?