Tutorial updates -- python version..

[krs-world]

The following tutorials have been updated, but some python parts are not working and needs to be looked at. All the code is in the testing repo. Follow. the sequence of teh C++ and Java versions for uniformity

tut_sllist_p3.py (this part is supposed to use iterators to illustrate traversal, but not working yet)
tut_dllist_p3.py ( same , but also include reverse iterator)