A. Joseph. (2019)cite arxiv:1912.10997Comment: 122 pages, 34 figures, several appendices. These lecture notes are based on the three lectures given at the 2019 Joburg School in Theoretical Physics: Aspects of Machine Learning, Mandelstam Institute for Theoretical Physics, The University of the Witwatersrand, Johannesburg, South Africa (November 11 - 15, 2019).