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 (https://github.com/swederik/nipype/commit/313d6a02ca6c156298a2a8494fabcc3fe7ec4e20)

Tensor mode screenshot. Linearly anisotropic (i.e. one direction) diffusion areas like the corpus callosum have high values (white), lower values are found in regions of crossing fiber bundles, indicating planar anisotropy.

![Screen Shot 2013-04-02 at 23 05 47](https://f.cloud.github.com/assets/607793/330988/0afeac42-9bdd-11e2-9e72-35386a4b2abb.png)