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?
Should we use a `bool` or `uint8` array here instead of a double array? We could coerce the input in the init to make for a more flexible API. My main concern with the way it is now is that if a user passes in something like `img_data == wm_class` they'll get an error.