def displace(x,y,z)
      Atom.new(self.x+x, self.y+y, self.z+z, self.species, self.constrain)
    end
def displace(x,y,z)
      Atom.new(self.x+x, self.y+y, self.z+z, self.species, self.constrain)
    end
An exact clone of the atom. Same ID and everything
 Return a new atom with the same species and relaxation constraints
 but with coordinates displaced by +x+, +y+, +z+