PCA Tutorial (pcatutorial.ipynb) could add a mathematical explanaiton

For the PCA Tutorial (pcatutorial.ipynb), I think it would be beneficial to add a mathematical explanation of how a PCA works. Go into to detail about the steps in a PCA (1. Standardization 2. Covariance Matrix 3. Computing eigenvalues and eigenvectors 4. Creating the feature vector). That way someone who has never learned about PCA before can have a better understanding of how it works. Sometimes it is really hard to understand something when the computer is doing all of the work for you. You could even consider adding a code section that manually goes through the math and validate the results using the PCA package in sklearn.