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?
No contiguous dimension specified--is this on purpose?