Fix issues in R CMD check

[qdread]

For the package to be accepted by CRAN it has to pass R CMD check or devtools::check() without any errors or warnings, and as few notes as possible. Some of the existing problems will go away once we finish the documentation but I need to fix the rest.

• [x] Fix "no visible global binding" note for ggplot2 things
• [x] ~Get rid of non-standard internal_docs folder~ instead added it to .Rbuildignore
• [ ] Document pitcher traits
• [x] Finalize which functions will be exported and which will not, and make sure all documentation is consistent in this regard
• [x] Make sure R version dependency is up to date so that sample data will not throw a warning

[qdread]

Closing this because it is redundant with #42