Seeding at the GM/WM interface

[arokem]

Our default seeding strategy right now is to seed throughout the wm with some fixed density. It may make sense to seed a lot more densely, but only at the GM/WM. If the inputs provide probabilistic maps of GM/WM it may be really straightforward to use these to find the interface and use that as a seed mask. If not, there could also be a way to calculate this interface from DWI using a combination of FA and MD.