Mendeleev

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* Enable visualizations of periodic tables with `plotly <https://plotly.com/>`_ as well as `bokeh <https://bokeh.org/>`_ backends
through ``mendeleev.vis.plotly.periodic_table_plotly`` and ``mendeleev.vis.bokeh.periodic_table_bokeh``
functions.
* Add ``mendeleev.vis.periodic_table`` function for convenient periodic table plotting wrapping both plotting
backends.
* Refactored the ``mendeleev.vis`` module so it can be wasily extended with plotting backends.
* Add ``CITATION.cff`` file.

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* Update ionic and crytal radii for III+ actinoids.
* Refactor electronegativity calculations for easier calculation and retrieval of the different scales.
* Add `fetch.py` module with methods for accessing bulk data.
* Add `oxides` methods to `Element` that returns possible oxides (`Issue 17 <https://github.com/lmmentel/mendeleev/issues/17>`_).
* Add tutorials on fetching data and electronic configuration.
* `tables.py` is renamed to `models.py`.
* Switch from `pipenv` to `poetry` for development.
* Switch from travis CI to github actions and extend testing matrix to Win and MacOS.
* Documentation udpate.

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* Add `electrophilicity` index.
* Pin `sqlalchemy` version to prevent further issues with old versions, see `Issue 22 <https://github.com/lmmentel/mendeleev/issues/22>`_

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* Add `Ion` class to handle atomic ions.
* Add Github templates for bug reports, feature requests and pull requests.
* Update the values of `atomic_radius_rahm` according to corrigendum, (`PR 13 <https://github.com/lmmentel/mendeleev/pull/13>`_).
* Switch the default documentation theme to material with `sphinx-material <https://github.com/bashtage/sphinx-material/>`_.

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* Fix a ``UnicodeDecodeError`` from README.md while installing on windows.
* Code quality improvements based on `lgtm.com <https://lgtm.com/projects/g/lmmentel/mendeleev/context:python>`_

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* Fix `issue 3 <https://github.com/lmmentel/mendeleev/issues/3>`_, ``get_table('elements')`` throwing an error