Release greta 0.5.1

[njtierney]

Prepare for release:

• git pull
• Check current CRAN check results
• Polish NEWS
• urlchecker::url_check()
• passes posterior checks
• devtools::check(remote = TRUE, manual = TRUE)
• devtools::check_win_devel()
• revdepcheck::revdep_check(num_workers = 4)
• Update cran-comments.md
• git push

Submit to CRAN:

• usethis::use_version('patch')
• devtools::submit_cran()
• Approve email

Wait for CRAN...

• Accepted 🎉
• usethis::use_github_release()
• usethis::use_dev_version(push = TRUE)

[njtierney]

greta was archived - I submitted a patch on Friday 19th, and am currently awaiting a reply from CRAN regarding an issue with a .keras directory appearing on test that I was not able to replicate locally, but I got some useful advice from Tomasz Kalinowski (author of keras and tensorflow R packages) for resolving this. I have not heard back from CRAN.