derivation: waveguides

 - https://www.desy.de/~njwalker/uspas/coursemat/notes/unit_2_notes.pdf
 - https://www.feynmanlectures.caltech.edu/II_24.html
 - https://web.mit.edu/6.013_book/www/chapter13/13.4.html

In my notes, see `ben/lab_notebook/appendix_q1d_transfer_matrix_derivation.tex`