Mendeleev

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* [DOC] Update docs for atomic scattering factor by :user:`lmmentel` in :pr:`203`
* [ENH] Add Miedema's scale of electronegativity by :user:`lmmentel` in :pr:`204`
* [ENH] Improve data export by :user:`lmmentel` in :pr:`205`
* [ENH] Update ionization energy data and metadata from NIST ASD by :user:`lmmentel` in :pr:`192`
* [FIX] Correct the electronic configuration for Lr by :user:`lmmentel` in :pr:`190`
* [FIX] Fix footnotes on data doc page by :user:`lmmentel` in :pr:`206`
* [FIX] Typo in changelog by :user:`lmmentel` in :pr:`196`
* [FIX] Update values of oxidation states by :user:`lmmentel` in :pr:`202`
* [MNT] Add dependency caching in CI by :user:`lmmentel` in :pr:`197`
* [MNT] Add pytest-xdist to dev deps and configure to use all cores by :user:`lmmentel` in :pr:`201`
* [MNT] Bump versions of dev dependencies by :user:`lmmentel` in :pr:`200`
* [MNT] Improve performance of `fetch_electronegativity` by :user:`lmmentel` in :pr:`198`

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* [MNT] drop pdf format from docs by :user:`lmmentel`

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* [ENH] Add atomic scattering factors by :user:`lmmentel` in :pr:`174`
* [DOC] Update data documentation by :user:`lmmentel` in :pr:`175`
* [DOC] Add api docs for the ``PhaseTransition`` model by :user:`lmmentel` in :pr:`181`
* [DOC] Add ``phasetransitions`` table to data access docs by :user:`lmmentel` in :pr:`183`
* [DOC] Correct the data export instructions by :user:`lmmentel` in :pr:`184`
* [DOC] Add metadata on ``PhaseTransition`` and update docs by :user:`lmmentel` in :pr:`185`
* [DOC] Update tutorial notebooks by :user:`lmmentel` in :pr:`187`

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* [ENH] Add data export to various formats by :user:`lmmentel` in :pr:`151`
* [ENH] Create metadata table for stored properties by :user:`lmmentel` in :pr:`156`
* [ENH] Update Isotope.half_life_unit values by :user:`lmmentel` in :pr:`160`
* [DOC] Add section for alternative implementation of mendeleev to README by :user:`lmmentel` in :pr:`163`
* [DOC] Add Mendeleev.jl to README by :user:`lmmentel` in :pr:`164`
* [FIX] Use read-only SQLite connection by :user:`jan-janssen` in :pr:`165`
* [FIX] Fix fetch_table to be compatible across major versions of pandas and sqlalchemy by :user:`lmmentel` in :pr:`159`

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* * Fix ``ImportError`` not being raised with missing objects by :user:`lmmentel` in :pr:`150`

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* [FIX] Fix import error by :user:`lmmentel` in :pr:`148`