add comments in canal.py
add comments in canal.py

demianw/dipy,Messaoud-Boudjada/dipy,StongeEtienne/dipy,jyeatman/dipy,jyeatman/dipy,Messaoud-Boudjada/dipy,samuelstjean/dipy,oesteban/dipy,rfdougherty/dipy,JohnGriffiths/dipy,samuelstjean/dipy,beni55/dipy,sinkpoint/dipy,rfdougherty/dipy,nilgoyyou/dipy,FrancoisRheaultUS/dipy,villalonreina/dipy,beni55/dipy,samuelstjean/dipy,StongeEtienne/dipy,mdesco/dipy,JohnGriffiths/dipy,sinkpoint/dipy,matthieudumont/dipy,FrancoisRheaultUS/dipy,nilgoyyou/dipy,villalonreina/dipy,mdesco/dipy,matthieudumont/dipy,oesteban/dipy,demianw/dipy