Upkeep

[olivroy]

• Rename tests files to align with R/ files (to improve workflow with use_r() and use_test())
• Update gh actions with usethis::use_github_action() to avoid deprecation warnings
• Fix typos
• Update URLs with urlchecker::url_update() + manual edits
• Remove master branch references
• Rebuild README with devtools::build_readme().
• Readjust indent in DESCRIPTION

[edzer]

Thanks!