NF - added anatomically-constrained tractography (ACT) tissue classifier
STYLE - addressed PEP8 comments, added references
STYLE - updated error msg
NF - added binary tissue classifier.
This allows a tracking in a predefined binary tracking mask.
doc/example/newapi.py shows a quick example.
BF - added a missing round() to map streamline point to voxel coordinates
BF - handle various dtype input in tissue classifier
TST - tests for tissue classifiers