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