Improving transparency in sysdata

[dbarneche]

• Ideally we should add the code that creates R/sysdata.rda for transparency sake, see example of code and folder structure in
  https://github.com/open-AIMS/bayesnec/blob/master/data-raw/sysdata.R

• Also, I note that some things seem to be for internal use only? gbr*_ , but others might be useful to the users? e.g. map.df. I would differentiate those into separate files and make datasets available as objects, with their own documentation. For example here, https://github.com/open-AIMS/bayesnec/blob/master/data-raw/sysdata.R, you'll notice that the usethis:: call at the end has the argument internal = TRUE. So, this is only visible to us internally. However, other datasets like this one
  https://github.com/open-AIMS/bayesnec/blob/master/data-raw/herbicide_data.R are supposed to be seen by the user, so we don't set it to internal only (and hence do not save it as part of R/sysdata.rda).

• Then you can also add a doco file in the R folder to explain what the data.frame is about:
  https://github.com/open-AIMS/bayesnec/blob/master/R/data.R

• I would also rename data objects to replace . for _