Fenwick Tree (#6319)

* Enhance fenwick_tree.py

* Change update to add in fenwick_tree.py

* Some changes

* Fix bug

* Add O(N) initializer to FenwickTree

* Add get method to Fenwick Tree

* Change tree in Fenwick Tree

* Add rank query to FenwickTree

* Add get_array method to FenwickTree

* Add some tests

* Update data_structures/binary_tree/fenwick_tree.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update data_structures/binary_tree/fenwick_tree.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update data_structures/binary_tree/fenwick_tree.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* change `List` to `list`

Co-authored-by: Christian Clauss <cclauss@me.com>