Add read_xyz and write_xyz functions

[Ruibin-Liu]

xyz file format is quite commonly used in quantum chemistry calculations and many molecule visualization programs like Avogadro.

The read_xyz function should take care of the case with multiple structures. I suggest use model_id as the column name.

It will be better if the comment line can be parsed as well. Check this page for hints to parse that line. Grouping this line and the number of atoms into a new DataFrame should be okay. model_id should be used as well if multiple structures are in the file.