Add examples to section Cartesian Product With Interactions

[worden-lee]

The Watson model is the primary, motivating example, introduced before this section: host population stratified into homophilous communities. In this section add additional examples such as host population stratified by gender, by high/low risk, pathogens stratified into types.

[worden-lee]

Hethcote & Yorke

Discussion of age structure and problem of stiff ODEs