Hamiltonian construction

This does several things:

• Switches the Hamiltonian format of builder_to_tb to dense because we expect no speedup due to needing full spectrum.
• Prohibits Kwant lattices without norbs set (the code used to assume 1 orbital, but this is not what norbs=None means).
• Avoids
  $\mathcal{O}(N^2)$
  or
  $\mathcal{O}(N^3)$
  operations for constructing the Hamiltonian, and therefore likely providing a speedup.

I did not modify the tests, and therefore I don't know whether they are specific enough.

assigned to @anton-akhmerov

changed the description

resolved all threads

approved this merge request

mentioned in commit 7028c68a

merged