RF: refactor remove_similar_vertices
RF: refactor remove_similar_vertices

Rename some variables, refactor return variable logic, add comments.

Logic almost the same, except for raising error for vertices list of
exactly length 2**16 instead of > length 2**16.

FrancoisRheaultUS/dipy,demianw/dipy,nilgoyyou/dipy,FrancoisRheaultUS/dipy,JohnGriffiths/dipy,villalonreina/dipy,sinkpoint/dipy,matthieudumont/dipy,matthieudumont/dipy,sinkpoint/dipy,StongeEtienne/dipy,nilgoyyou/dipy,jyeatman/dipy,JohnGriffiths/dipy,StongeEtienne/dipy,jyeatman/dipy,villalonreina/dipy,demianw/dipy