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?
IIRC This attribute is passed to a function which takes memory views as inputs. A pointer cannot be used here because pointers have not shape info and cannot be coerced into memory-views. An c array does work, but when I profiled it, I noticed that coercing a c array into a memory view adds a lot of overhead. If we can just do it once up front, it makes a noticeable difference in performance.