Add species-specific Beta-2-microglobulin

We have upcoming uses for species-specific Beta-2-microglobulin proteins. For most species there will be one, but @rvita has a case where there are two. We agreed to use the common name: e.g. "clawed frog Beta-2-microglobulin". They should all have sequences.

I would prefer to update the `molecules.tsv` table with the species-specific Beta-2-microglobulin in the Beta Chain column.