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 Jordan axiom and norm tests once all of the
   algebras pass.

6. Create Element subclasses for the matrix EJAs, and then override
   their characteristic_polynomial() method to create a new algebra
   over the rationals (with a non-normalized basis). We can then
   compute the charpoly quickly by passing the natural representation
   of the given element into the new algebra and computing its charpoly
   there. (Relies on the theory to ensure that the charpolys are equal.)