Release 1.3.0: crystal indexing and GPLv3

This release brings one exciting change: the implementation of crystal structure indexing!

• General purpose single-crystal structure indexing with the DirAx algorithm has been added.

In the future, more algorithms are expected to be added.

Other notable changes:

• Starting with this version, crystals is licensed under GPLv3.
• Lattice.scattering_vector and Lattice.miller_indices now accept tables of reflections/scattering vectors. This calculation is vectorized using NumPy.
• Migration of testing infrastructure to pytest.
• Support for Python 3.6 and NumPy<1.17 has been dropped

Release 1.2.2

• The sorting of AtomicStructure objects is now stable.
• Fixed an issue where checking the containership of atoms did not work properly.
• Fixed an issue where downloading structures from the Materials Project failed on macOS/Linux.
• Code snippets in documentation are now tested for correctness.
• Tests are now included in the package itself.

Release 1.2.1

This release fixes some deprecation warnings that appeared with Python 3.9.

Release 1.2.0

Release 1.2.0 sees the following changes:

• 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.