The broadening matrix (gamma) itself has norm ~0 when there are no
open modes, meaning that the tolerance we are checking against is
always too small in this case. Instead we check against the norm
of the selfenergy, which is nonzero even when there are no open modes.

A global constant named 'chain' is defined later in the test module,
so 'from itertools import chain' is confusing.

Describe more fundamental attributes first.

fix eradicate dangling across translational symmetry

See merge request kwant/kwant!369

Allow mode-space solvers to work with systems that have self-energy leads attached

Closes #368

See merge request kwant/kwant!366

We cannot, of course, calculate scattering wavefunctions that
*originate* in a selfenergy lead, as that would require knowing
the modes. For this reason the test was changed from checking
for 'NotImplementedError' to 'ValueError'

This transformation is a pure refactor. It was not done in the previous
commit so that the relevant change would be clear.

the weird 'if: x; continue' idiom is just to keep the changeset
small and relevant. The next commit will fix this by indenting
the existing code into an 'else' clause.

It is meaningful to calculate scattering matrix elements for
leads that are specified as modes, but not for leads specified
as selfenergies only.

smatrix should "just work" as long as we are not
requesting scattering amplitudes to/from a selfenergy lead.
Similarly wave_function should "just work" as long as we
are not asking for a wavefunction originating from a
selfenergy lead.

Add a clarification to the self-energy calculation for leads

See merge request kwant/kwant!353

Vectorize wraparound

See merge request kwant/kwant!363