Merge branch 'week8-robert' into 'week8'

book/_toc.yml

book/external/learn-programming

book/external/learn-python

book/multivariate/Introduction.md

book/multivariate/events.md

book/multivariate/figures/venn-events.png

book/multivariate/figures/venn-intersection.png

book/multivariate/functions.md

book/multivariate/gaussian.md

book/multivariate/nongaussian.md

book/multivariate/overview.md

book/multivariate/variables.md

book/programming/week_1_7.md