Merge branch '39-set-as-data-frame-method-for-prepgr-calgr-and-simgr-classes' into 'dev'

Resolve "Set as.data.frame method for 'PrepGR', 'CalGR' and 'SimGR' classes"

Closes #39

See merge request !16
3 jobs for dev in 5 minutes and 36 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Check
passed check_as_cran_devel #178023

00:01:52

passed check_as_cran_oldrel #178024

00:01:36

passed check_as_cran_patched #178022

00:02:07