Add methods to load a material from a CIF file

[stefsmeets]

This PR adds a function to load a material from a cif file. These get loaded into a Structure. The coordinates not symmetrized.

By extension, I added a way to add known materials to GEMDAT to mimic known_materials.md. To make a super cell, we can conveniently use Structure.make_supercell.

Known materials are stored in src/data.

Closes #17

Load cif file

from gemdat.io import load_cif

s = load_cif('path/to/argyrodite.cif')

Load known material

from gemdat.io import load_known_material

s = load_known_material('argyrodite')

# making a super cell (double along a)
s.make_supercell([2,1,1])