Warn the user if they try and calculate the Smatrix at an energy where no modes are open
Often we get questions on the mailing list where people get 0 transmission and don't understand why. The reason is often because they have not checked the bandstructure and are trying to calculate the scattering matrix at energies where there are no propagating modes.
In order to avoid this class of mistakes, we could warn a RuntimeWarning
if the user requests
the smatrix at energies with no modes open