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).
`self.assertEqual` should work here. Alternative the `numpy.testing` module provides independent assert methods (that can be freely used in `unittest` testcases) for array like objects.