feat: seemless integration of airGR into airGRiwrm

- import airGR with "Depends"
- remove all library(airGR) and airGR::FUN

Closes #63
2 jobs for 63-simplify-the-use-of-airgr-in-airgriwrm in 8 minutes and 2 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Checks
passed check_as_cran #219686
docker

00:08:01

passed check_not_cran #219685
docker

00:07:14