Sankey plot

[mabudz]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

This PR introduces a function sankey() to create a basic Sankey diagram using plotly [https://plotly.com/python/sankey-diagram/]. This figure can be further modified (e.g. fig.update_traces()) or used (e.g. fig.write_html()) by the user [https://plotly.com/python/graph-objects/]

The function uses a mapping dictionary {'variable1': ('source1', 'target1'), ...} to specify the source and the target node for the considered variables.

Currently only one region and one year can be plotted.

[danielhuppmann]

there was an update of the sphinxcontrib-bibtex package and a change of the WorldBank data used in the pandas_datarader test, which currently causes unit tests to fail - don't worry about that for now

[mabudz]

Alright. I just address the recommendations of picky Mr.Sticklers

[danielhuppmann]

thanks @mabudz for this nice addition! a few suggestions to clean up the implementation... Also, I resolved all issues by stickler that you have implemented, but there are still a few stickler requests...

[danielhuppmann]

I also just realized that you need to add plotly to the dependencies explicitly, i.e. here

[mabudz]

Ah, I forgot to add the plotly dependencies. Should not do this while sitting on the airport :)))

[danielhuppmann]

One minor suggestion for a formatting clean-up inline, and please add to the release notes - then good to go! As discussed, we'll tackle an example for the plotting gallery and tests during/after the holidays...

[danielhuppmann]

Merging and tackling the tests in a next iteration - thanks @mabudz!