0. Add tests for orthogonality in the Peirce decomposition.

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. Can we make the minimal and characteristic polynomial tests work
   for trivial algebras, too? Then we wouldn't need the "nontrivial"
   argument to random_eja().