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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/>`_.