changes to lessons three & four

[libjohn]

PR for issues 13-21, 29 changes cover data wrangling and visualization with a bias towards Tidyverse

• https://github.com/LibraryCarpentry/lc-r/issues/13
• https://github.com/LibraryCarpentry/lc-r/issues/14
• https://github.com/LibraryCarpentry/lc-r/issues/15
• https://github.com/LibraryCarpentry/lc-r/issues/16
• https://github.com/LibraryCarpentry/lc-r/issues/17
• https://github.com/LibraryCarpentry/lc-r/issues/18
• https://github.com/LibraryCarpentry/lc-r/issues/19
• https://github.com/LibraryCarpentry/lc-r/issues/20
• https://github.com/LibraryCarpentry/lc-r/issues/21
• https://github.com/LibraryCarpentry/lc-r/issues/29