Mendeleev

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* Migrate the package from bitbucket to github
* Add Pettifor scale: ``pettifor_number`` attribute
* Add Glawe scale: ``glawe_number`` attribute
* Restore default printing of isotopic abundancies, fix issue 9
* Correct the oxidation states for Xe, fix issue 10

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* Update dipole polarizability value to the latest recommended (2018)
* Fix `issues/8/typeerror-on-some-of-the-element <https://bitbucket.org/lukaszmentel/mendeleev/issues/8/typeerror-on-some-of-the-element>`_

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* Fix `issues/6/type-of-block-is-wrong <https://bitbucket.org/lukaszmentel/mendeleev/issues/6/type-of-block-is-wrong>`_

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* Added ``mendeleev_number`` attribute to elements.
* Added footnotes to the data documentation.

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* Fixed issue 3: encoding issue in econf.py.

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* Corrected passing integers to the CLI script.
* Various documentation readability and structure improvements.