ASKurz/Statistical_Rethinking_with_brms_ggplot2_and_the_tidyverse: Correct multinomial and Gaussian process models

Noteworthy changes include:

• the correct solution to the first multinomial model in Section 10.16;
• a coherent workflow for the Gaussian process model from Section 13.4;
• corrections to some of the post-processing workflows for the measurement-error models in Section 14.1;
• refitting all models with the current official version of brms, version 2.13.5;
• improved in-text citations and reference sections using BibTex, Better BibTeX, and zotero; and
• minor prose, hyperlink, and code edits throughout.