Add reference for documentation on surface participation ratios

[sebastiangrimberg]

These formulas come from https://pubs.aip.org/aip/apl/article-abstract/99/11/113513/923377/Surface-loss-simulations-of-superconducting?redirectedFrom=fulltext.

[sebastiangrimberg]

Related, add a note to the first paragraph of reference.md about nondimensionalization:

This nondimensionalization will be used throughout the entire reference.

[sebastiangrimberg]

Also a (new) reference for wave ports: Vardapetyan and Demkowicz, Full-wave analysis of dielectric waveguides at a given frequency, Math. Comput. (2003).

[sebastiangrimberg]

Closed by #189