feat(S3): set as.data.frame method for 'PrepGR', 'CalGR' and 'SimGR' classes

- remove airGRt class
- rename as.data.frame.airGRt files into as.data.frame
Refs #39
1 job for 39-set-as-data-frame-method-for-prepgr-calgr-and-simgr-classes in 1 minute and 53 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Check
passed check_as_cran_patched #177449

00:01:53