ENH: add build interface to manip. atoms tutorial

ENH: add build interface to manip. atoms tutorial

The manipulating atoms tutorial (manipulating_atoms.rst) is extended with a little tutorial to add a layer of water to a surface. Two helper python files are included. WL.py creates WL.traj, which can be imported using ase.io.read. interface-h2o.py loads WL.traj and creates the png files for the tutorial.