[COMPLETE] Theory section of presentation

[smu095]

TO DO:

• [x] Bag of marbles example (borrow liberally from McElreath)
• [ ] Transition to Bayes theorem in parameter estimation context
• [x] Motivate MCMC
  • [x] Brief (!) introduction to Markov chains
  • [x] Brief (!) introduction to Monte Carlo methods
  • [x] Brief (!) explanation of MCMC
• [x] Add brief slide about PyMC3
• [X] Add useful links/literature for the interested
• [x] Add explicit definitions to prior/likelihood/posterior slides
• [x] Optional: Maybe combine slides 6-7?