1. Add CartesianProductEJA.

2. Check the axioms in the constructor when check != False?

3. Add references and start citing them.

4. Implement the octonion simple EJA.

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

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

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