TST: added test for _to_voxel_tolerance.
TST: added test for _to_voxel_tolerance.

JohnGriffiths/dipy,FrancoisRheaultUS/dipy,StongeEtienne/dipy,villalonreina/dipy,matthieudumont/dipy,nilgoyyou/dipy,nilgoyyou/dipy,FrancoisRheaultUS/dipy,JohnGriffiths/dipy,StongeEtienne/dipy,matthieudumont/dipy,villalonreina/dipy