NF - Deterministic Maximum Direction Getter

New direction getter similar to the deterministic tracking of mrtrix.

- do not follow peaks, but the maximum value of the pmf
- i.e. probabilistic direction getter returning the maximum value of the distribution instead of drawing from it
- pretty easy to add with this design @MrBago! 

*whoop with the commit, I forgot one commit, so the RF - ranamed.. should be NF - new direction...

I want to rename the file "dipy/direction/probabilistic_direction_getter.py" to "dipy/direction/direction_getters.py". Should I do it here, or in another PR?