double_linear_search algorithm (#2161)

* linear search iterating from both array sides

* Update double_linear_search.py

* Update double_linear_search.py

* added doctests

* updated doctests

* Update double_linear_search.py

* Update double_linear_search.py

* added blank after >>>

* made all the requested changes

* Update double_linear_search.py

* Update double_linear_search.py

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