New compression format: xz (instead of bz2)

[coreyoses]

AFLOW has recently adopted a new default compression format: xz.
Generally, any distributed compressed file (.gz or .bz2) has been newly compressed with xz, and thus exists with the extension .xz.
This includes the various AFLOW versions: http://materials.duke.edu/AFLOW/aflow.3.1.197.tar.xz.

For more information on this new compression format, see here: https://tukaani.org/xz/format.html.