1. Implement the SPD cone.

2. Make it work on a cartesian product of cones in the correct order.

3. Make it work on a cartesian product of cones in the wrong order
   (apply a permutation before/after).

4. Add random_game() and use it to replace all of the tests where
   the cone is irrelevant. This should be done only after we are
   feature complete and sure that everything works.

5. Remove the one use of numpy.matrix in a doctest, since that's
   an annoying dependency to have just to run the test suite.