Crystals

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* Added the ability to write crystal structures to CIF/XYZ files with the `Crystal.to_cif` and `Crystal.to_xyz` methods. Also, structures can be converted to ASE's ``Atoms`` class with `Crystal.to_ase`. This can be used to convert crystal structures from one format to another!
* Added the `symmetry_reduction` function, and associated method `Crystal.asymmetric_cell()`, to determine the smallest asymmetric cell that generates a unit cell.
* The method `Lattice.bounded_reflections` now takes in an additional parameter, `min_bound`, to find reflections between a lower and upper bound.
* Fixed an issue where in certain cases, atoms related by symmetry were not pruned appropriately (5)
* Official support for Python 3.9.

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* Atom site occupancies are now parsed from CIF files `(3) <https://github.com/LaurentRDC/crystals/issues/3>`_.

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* ``Orbital`` instances are now orderable, i.e. ``Orbital("1s") < Orbital("2p")``.
* The ``Element`` class can now be initialized from another ``Element``, and also from full atomic names (e.g. ``"hydrogen"``). This simplifies the normalization for input types.
* Added the ``ElectronicStructure.outer_shell`` method to quickly get the outermost shell.

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* Added the ability to describe the electronic structure of atoms using the ``ElectronicStructure`` class.

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* Added the ability to set a Materials Project API key as environment variable ``MATERIALS_PROJECT_API_KEY``.
* Added the Materials Project constructor to the ``crystals info`` script.

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* Added the ``Crystal.from_mp`` constructor to create crystals through the Materials Project API.
* Added the method ``Crystal.indexed_by``, which allows to index reflections of a crystal by using another lattice.
* Re-organized the ``Crystal`` class hierarchy to be more specific. This change should not affect anyone.