Another method added for GCD (#1387)

* Another method added for GCD

* Now doctest fulfilled for added method.

* Update greatest_common_divisor.py

* Now unnecessary white spaces removed.

* Cycle_Detection_Undirected_Graph

Cycle_Detection_Undirected_Graph using Disjoint set DataStructure

* Update greatest_common_divisor.py again

* Again Updated cycle_detection_undirected_graph.py

* Delete cycle_detection_undirected_graph.py

* Add doctests and format the code with psf/black

* fixup: Typo

* Update greatest_common_divisor.py

* greatest_common_divisor()