Add sections on fast/slow code paths and which matrix classes work with which path, on memory use, and other internals.

The iris data for LDA / classification is overused and typically mis-applied [1].

Let's use a new data set for our LDA examples and include it in the package. Steinfurth et al. have a paper on classifying penguins by sex using various body measurements [2] which seems like it would make a great example.

Idea from [3]; see also [4-5].

[1] http://www.dicook.org/files/jsm19/slides#1