Extend SLH class to allow for description of port names and optionally keep track of eliminated internal modes

[ntezak]

It would be quite useful to store more metadata with an SLH object, such as the labels of inputs and outputs as well as any internal signals that were eliminated through feedforward/feedback. Ideally we would also store scattering matrices and L operators for the internal modes, which can then be used to compute transfer functions from external input to some internal mode, and to give the expected mode amplitudes for internal modes. The QHDLJ package I wrote supports this through the ABCD formalism as well. We still need to figure out if this can be generalized from this almost linear case.