Updated Implementation of Roberts Algorithm
Upadated Version with test added(/filter/tests/test_edge.py
updated tests for Roberts Edge Detector
Updated function names[Robert's Edge Detection]
Final Roberts Edge Detector
Updated Test Case
Minor cleanups

* PEP8 clean ups (space after commas, docstring spacing, prefer parentheses over line continuation)
* Simplify creation of test images.
* Use `plt.subplots` instead of state machine
* Minor wording modifications
Merge branch 'pr/517'