read cg tables

[RWParsons]

In this branch I have:

• made read_correspondence_tbl() which reads the correspondence tables from {absmapsdata} in the same way as read_absmap() does. I include tests for this similar to those for read_absmap() and added myself as a ctb.
• incremented the pkg version to a development version and noted the above change in the NEWS.md file.

Currently, the absmapsdata package stores all correspondence files together in the same sysdata.rda (https://github.com/wfmackey/absmapsdata/blob/master/R/sysdata.rda) file on GitHub. This means that read_correspondence_tbl() downloads all of these and then picks out the one that's requested. This isn't particularly efficient, though. If we wanted to make this a bit more streamlined, I could make a change to absmapsdata so that each of the correspondence tables are stored in separate .rda files and then only the selected one is downloaded and returned to the user. I don't think this is a biggie as I doubt people are repeatedly loading correspondence files in their workflows and the sysdata.rda file is only ~15MB. BUT, if you think this is preferable, I'm happy for this pull request to be rejected and make the change in absmapsdata.

Thanks, Rex

[wfmackey]

Thanks Rex, this looks good.

Agree that it would be cleaner to be able to download individual correspondence tables rather than the whole batch (an unexpected downside of the way I have structured it in absmapsdata). But, given the manageable file size and the ability of the user to download and store the file at a specified path (rather than download each time), I think this is okay for now.

If it becomes a hassle in the future, we can lodge an issue and fix it on the absmapsdata side.