Added an example of an XYZ file with 2 molecules

This is a simple example of an XYZ file containing two molecules, they
can also be used to give multiple frames in a trajectory (same molecule,
different coordinates).

Change-Id: Ib38d4065e2e41954270eb39ca793b5cb933f1005
Merge topic 'multi-xyz' into master

39921267 Added an example of an XYZ file with 2 molecules