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().

7. Pass already_echelonized (default: False) and echelon_basis
   (default: None) into the subalgebra constructor. The value of
   already_echelonized can be passed to V.span_of_basis() to save
   some time, and usinf e.g. FreeModule_submodule_with_basis_field
   we may somehow be able to pass the echelon basis straight in to
   save time.