Add an XYZ format reader/writer.

I'll need this for obabel bond perception.

The new test will fail without the new methane.xyz file
in avogadrodata.

Change-Id: Ib44686aa475e61210b4a456067e4e8a6510fc0a3
Merge topic 'xyz-format' into master

2377c14e Add an XYZ format reader/writer.