ENH: Miller indices for primitive cells

[ajjackson]

When a primitive cell is provided (as is typical for phonon calculations) the lattice vectors are usually not aligned with the conventional cell vectors. The .ascii format discards this information by using a 6-value unit cell description, but it might be recovered using the standard primitive cell vectors of a spacegroup.

Pros

More intuitive "Miller indices" can be used, default alignment will look better for primitive cells

Cons

Added spglib dependency (or need to integrate/re-implement spglib)