Add precision argument to yr_transform()

[Joe Roe]

yr_transform() is precise with epoch transformations, which is appropriate for more recent years but unnecessarily messy for distant ones:

library("era")
yr(100000000, "BCE") %>% 
  yr_transform(era("mya"))
#> # mya years <yr[1]>:
#> [1] 100.0019
#> # Era: million years ago (mya): calendar (×1e+06) years, counted backwards from 1950

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

It would be nice to add an option to round the output to a desired level of precision in yr_transform().