1. Add references and start citing them.

2. Profile (and fix?) any remaining slow operations.

3. Every once in a long while, the test

       sage: set_random_seed()
       sage: x = random_eja().random_element()
       sage: x.is_invertible() == (x.det() != 0)

   in eja_element.py returns False. Example:

       sage: J1 = ComplexHermitianEJA(2)
       sage: J2 = TrivialEJA()
       sage: J = cartesian_product([J1,J2])
       sage: x = J.from_vector(vector(QQ, [-1, -1/2, -1/2, -1/2]))
       sage: x.is_invertible()
       True
       sage: x.det()
       0

4. When we take a Cartesian product involving a trivial algebra, we
   could easily cache the identity and charpoly coefficients using
   the nontrivial factor. On the other hand, it's nice that we can
   test out some alternate code paths...