Separate dwi code into separate python module?

[mhe]

The dwi branch contains code contributions by @hjmjohnson and @aghayoor to support a specific type of nrrd files containing DWI data. In this dwi branch I attempted to include this code in pynrrd, but in the end I decided that doing so would add unnecessary complexity and I would like to keep pynrrd as straightforward and simple as possible. I propose this dwi code to be moved into a separate python module hosted elsewhere.

Questions:

• Who is using this DWI code?
• Who would be willing to maintain this DWI code?
• Other thoughts?