Feature lattice

Implementation includes
- class LatticeNode: represents a single node of a lattice,
- class Lattice: describes a Bravais lattice (stores references to lattice nodes),
- factory routines for creating specific lattices (for convenience).
If this is the docstring for LatticeNode, I would switch *Parameters* to *Attributes* and update *lat_node* to *data*.  

It might be that this docstring was intended to document the __init__, if so then it should be moved to that method.