* Add an implementation for interval_graph and its unit tests

* Fix violations of pep8 convetions for #3705

* Fix PEP8

* reflect comments for #3705

* Fix PEP8 and remove docstring tests

* fix PEP8

* minor changes in style

None is not checked as input. validation done inline.
test graphs created quicker. added isolated node to one test.

* Add test and correct logic of overlapping intervals

* minor formating of tests and doc

* last code tightening. should be ready

* Fix the bugs I just introduced...

* sorted the edges so all systems pass test

Co-authored-by: Dan Schult <dschult@colgate.edu>

* Add Sudoku graph generator

* Remove doctest and create_using parameter

* fix typos

* rewrite sudoku tests as a class

* improve code style

* fix typo in docstring

* fix typo in docstring

* Add reference

* Parameterize tests for sudoku_graph

* remove __main__

* Reverting change to sudoku_graph.py

* remove __main__ from sudoku_graph.py

* Improve documentation of sudoku submodule

* add internet_as_graph generator

This random graph generator implements a state-of-art eBGP speaker model derived from internet traces.
The resulting graph resembles real-world autonomous system networks.

* add compatibility with python 3.5, fix test precision, docstrings and style

* Create directed_joint_degree_seq.py

* Update __init__.py

Adding directed joint degree seq to namespace

* Refactoring code

Movde new functions from directed_joint_degree_seq.py to joint_degree_seq.py and test_joint_degree_seq.py

* Docs and style update

* * Implements full join operation and a generator for cographs.

* * Adding myself as a contributor.

* * Fix full join description.

* * Removes full_join function from the cograph generator file.

* * Changes ^ for **.

* Docstrings improvements, fixes and new tests.

* Includes new features in the docs.

* Fix typo in test_cograph.py.

* Random cograph docstring improvements.

* Adds support for directed graphs.

* Improvements to the full join operator and new tests.

* Fixed rename.

* Fix test description.

* Implements the binomial tree graph generator.

* Improvements to the binomial tree generator and Notes section.

* Remove Notes section from the binomial_tree and moves the info to the line under the first line of the doc_string.

* Move LFR_benchmark to generators

* Correct import line in docstring

* Removed LFR from algorithms.community.rst and put in generators.rst

Fixes #3404 

* Update generators.rst

Add the reference for the Harary graph generators.

* Update CONTRIBUTORS.rst

Add Weisheng Si as a contributor