Prepare for a CRAN submission

[cpauvert]

Resources to read:

• [x] https://r-pkgs.org/release.html
• [x] https://github.com/ThinkR-open/prepare-for-cran
• [x] https://github.com/DavisVaughan/extrachecks

Issues to solve:

• [x] hide non standard files like codecov
• [x] CITATION in inst (consider building the CITATION.cff from {cffr} https://docs.ropensci.org/cffr/ ?)
• [x] solve the qpdf issue
• [x] adjust proper email
• [x] add contributors
• [x] try out installation via rhub https://r-hub.github.io/rhub
• [x] #20
• [x] #21
• [x] #22
• [x] #24