Interface for Tensor Mode calculation from Dipy

Work in progress to get the mode of the diffusion tensor into an interface in Nipype.

See the Dipy pull request for more info:
https://github.com/nipy/dipy/pull/164

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.

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)