Test/honeycomb

[evanberkowitz]

Adds spectral tests to the honeycomb lattice. In particular

• The eigenvectors of the hopping matrix are orthogonal.
• The eigenvalues have 4 zeros if L1 == L2 == 0 (mod 3).
• The eigenvalues are bounded in [-3, +3].

[evanberkowitz]

@luutom Do you have any other ideas for spectral tests of the honeycomb?

[evanberkowitz]

@luutom if you can compile test_honeycomb on your laptop (and it passes) I will merge this PR.