Renaming of streamlines tractography algorithms

Some streamlines algorithms have had their names changed (both in the code and the tckgen command interface) to better reflect the underlying operation of these algorithms:
- 'FACT' has been renamed to 'Tensor_Det'.
- 'WBFACT' has been renamed to 'Tensor_Prob'.
- 'VecStream' has been renamed to 'FACT'.
Closes #39.