Submit 3.1.0 to CRAN

• Review CRAN Repository Policy
• Push all changes to GitLab.
• rhub::check_for_cran() and wait for all results.
• rhub::check(platform = c("macos-highsierra-release", "macos-highsierra-release-cran"))
• Run GitLab-CI.
• Update cran-comments.
• Check NEWS.md
• Check DESCRIPTION