update 'WaveFunction' to work when some leads are selfenergy leads

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'
22 jobs for feature/selfenergy-lead in 13 minutes and 21 seconds (queued for 3 seconds)