era arguments should always accept a character label

[Joe Roe]

Currently:

library("era")
yr(1, "BP")
#> # BP years <yr[1]>:
#> [1] 1
#> # Era: Before Present (BP): calendar years, counted backwards from 1950

But:

yr_set_era(1, era("BP"))
#> # BP years <yr[1]>:
#> [1] 1
#> # Era: Before Present (BP): calendar years, counted backwards from 1950

^{Created on 2020-11-12 by the reprex package (v0.3.0)}

This as annoying. Every function that has an era argument should also accept a character label to look up in eras().