Suggestion: import Orthodont data

[seaCatKim]

As written, the text seems to imply that the Orthodont data is from the lme4 package. Suggest moving and visibly reading in the data from the nlme package before the first linear model in section 7.4.1 to avoid confusion.

[seaCatKim]

Similar suggestion for the survival model later in the same section - not clear where the ovarian data set comes from. Seems to come from the survival package: data(cancer, package = "survival")?

[juliasilge]

Thanks for this PR @seaCatKim! 🙌 We are in the process of final printing for the first edition of this book but will look at this after we get that edition off and finished.

[presnell]

Came to make the same comment: data(Orthodont, package = "nlme") is needed before using the Orthodont dataset in section 7.4.1 (Special formulas and inline functions). The ovarian dataset is automatically loaded with the survival package, so you're arguably ok there since you already have library(censored) which loads the survival package.