1. Add CartesianProductEJA.

2. Add references and start citing them.

3. Implement the octonion simple EJA.

4. Factor out the unit-norm basis (and operator symmetry) tests once
   all of the algebras pass.

5. Override inner_product(), _max_test_case_size(), et cetera in
   DirectSumEJA.

6. Switch to QQ in *all* algebras for _charpoly_coefficients().