Hacktoberfest 2020: Added computer vision algorithm (#2946)

* Create meanthresholding.py

* Rename meanthresholding.py to meanthreshold.py

* Update meanthreshold.py

* Update computer_vision/meanthreshold.py

Verified this part works, thanks.

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

* Update computer_vision/meanthreshold.py

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

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