Release inlabru 2.3.1

[finnlindgren]

Prepare for release:

• [x] Check current CRAN check results
• [x] Polish NEWS
• [x] devtools::build_readme()
• [x] urlchecker::url_check()
• [x] git flow release start 2.3.1
• [x] usethis::use_version('patch')
• [x] Activate release branch checking. Push to github for checking; git flow release publish 2.3.1
• [x] devtools::check(remote = TRUE, manual = TRUE)
• [x] devtools::check_win_devel()
• [x] rhub::check_for_cran() (System dependencies for rgdal are broken on R-hub. Skip this Check)
• [x] Local R CMD build & check --as-cran with R-4.0.4 , R-devel, and R-3.6.2
• [x] revdepcheck::revdep_check(num_workers = 4)
• [x] Update cran-comments.md
• [x] git flow release finish 2.3.1
• [x] usethis::use_dev_version(), commit & push
• [x] git checkout stable, push and push tags

Submit to CRAN:

• [x] devtools::submit_cran()
• [x] Approve email

Wait for CRAN...

• [x] Accepted :tada:
• [x] usethis::use_github_release()
• [x] Tweet