Release volcalc 2.1.0

[Aariq]

Note: I edited this to remove CRAN-specific things, but maybe this is the version that gets submitted to CRAN?

After #72 & #73 is merged

• Proofread Title: and Description:
• Check that all exported functions have @return and @examples
• Check that Authors@R: includes a copyright holder (role 'cph')
• Check licensing of included files
• Update repostatus badge and remove "warning" from README

Prepare for release:

• Draft blog post
• git pull
• urlchecker::url_check()
• devtools::build_readme() (this doesn't work because of a bug, but also I don't think it matters if this isn't going on CRAN)
• devtools::check(remote = TRUE, manual = TRUE)
• devtools::check_win_devel()
• usethis::use_version('minor')
• Increment version in CITATION.cff and CITATION
• git push

Make release:

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