NF - added anatomically-constrained tractography (ACT) tissue classifier

Anatomically-Constrained Tractography (ACT) from Smith et al. (neuroimage 2012).

I didn't tested it yet with actual white/grey/csf partial volume fraction maps. I will run FSL/Fast on the B0 image of the stanford_hardi dataset to get some maps. Maybe we could added those image to the dataset at some point.

Do we have a T1 image of the stanford_hardi brain?
ya this is on purpose, I didn't want to require folks to copy their arrays in order to use this class. For example if a user has an FA image from a nifti file, it's going to be F contiguous (it's also likely to be float32 so that's maybe that's not the best example). Anyways, it makes very little difference all things considered so I wanted to leave it flexible.