Minor notes to self

[LTLA]

• [x] Comment on the relationship between the entropy and library size.
• [x] Demonstrate how to use rowSubset() for HVG selection.
• [x] Add a "comparing clustering section", with clusterRand and coassignProb.
• [x] Add a demonstration of clusterPurity as another cluster diagnostic.
• [x] Truncate the SingleR section and refer people to the other book.

[LTLA]

• [x] Comment on slingshot's robustness to the MST problem.
• [x] Mention the assumptions required to derive pseudotimes from velocities.