Calculation for mode of a tensor

Hello!

My first PR to Dipy: calculating the "mode" of a tensor.

From a 2006 MRM paper (http://lmi.bwh.harvard.edu/papers/papers/ennisMRM06.html):

> The mode of anisotropy is useful for resolving whether a region of anisotropy is linear
> anisotropic, orthotropic, or planar anisotropic"

It has also proven useful for identifying changes in regions of crossing fibers (http://www.ncbi.nlm.nih.gov/pubmed/21182970).

I've implemented it in Dipy based on the equations in the Ennis 2006 paper. I have a Nipype wrapper that will be sent as a PR as well.