Add multiple b0 slices to medotsu4D. Fix other minor review comments.
Add multiple b0 slices to medotsu4D. Fix other minor review comments.

medotsu4D is now able to take a single int OR a tuple of b0Slice indexes.
If multiple slice indexes are provided, the mean value of all slices is used.

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