Crystals

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* Added command-line utilities, including the `crystals info` script.

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* New algorithm to determine the lattice vectors from lattice parameters. Results should be the same except in edge cases.
* Space-group and symmetry information has been inserted into immutable dictionaries
* Fixed an issue where the parsing of CIF files without Hall symbol would fail because the International Table number was parsed as a string, not an integer.
* Updated minimum dependency bound on PyCifRW to 4.4.1 because of potential issues with Python 3.7.
* Refactoring of internal string handling to use f-strings.

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* ``Crystal.ideal`` and ``Crystal.primitive`` will now preserve subclasses.
* ``Crystal.primitive`` will now always return a new ``Crystal``, even if the ``Crystal`` already has a primitive cell.
* ``Supercell`` is no longer a subclass of ``Crystal``. The only recommended way of building supercells is now ``Crystal.supercell``.
* ``Lattice.scattering_vector`` and ``Lattice.miller_indices`` now work on single vectors of shape (3,).
* Added the ``Lattice.bounded_reflections`` generator.

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* Added the constructor ``Crystal.from_pwscf`` to create crystal instances from output files generated by the `Plane-Wave Self-Consistent Field (PWSCF) program <https://www.quantum-espresso.org/Doc/pw_user_guide/>`_.
* Added the ``Crystal.ideal`` method to create idealized crystal structures.
* Added the ``Crystal.reciprocal_symmetry_operations`` method to determine crystal symmetry operations in reciprocal basis.
* Symmetry-determination via ``Crystal.symmetry()`` and related methods now raises a ``RuntimeError`` if symmetry-determination fails, rather than returning ``None``
* Added the ``Atom.tag`` property, which can be used to tag ``Atom`` instances with extra information. This is for internal use. So far, it is used to label the order of Atoms from a PWSCF output file.

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* Added the ``Crystal.symmetry_operations`` method to determine crystal symmetry operations.

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* Fixed an issue where cache directories on MacOS could not be used.
* `CODParser` will now try to download files from three Crystallography Open Database mirrors.
* `AtomicStructure` and subclasses now support "truthiness", i.e. they are considered `False` if empty, and `True` otherwise.
* Added the `AtomicStructure.satisfying` method, to extract atoms satisfying a predicate from structures
* Added the `is_element` function. It can be used to make `AtomicStructure.satisfying` more readable.