Band structure calculations for an optical lattice

Initial release.

Here I present code to construct the single particle eigentstates in an
sinusoidal potential, such as that created by an optical lattice. We define
methods to calculate the band structure and implement interpolation functions
of the obtained results to speed up subsequent calculations.

The Wannier states in the lattice are constructed starting from the eigen-
states of the Hamiltonian.

The tunneling matrix element and the on-site interactions, relevant for the
Hubbard model are calculated.