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?
Yes, we need to address this. In this case (btc), a `bool` array will make perfect sense. The problem also appears with other tc, if the input is `float32` instead of `float64` for instance. 
Should we transform every input to `float64` in the init?