eja: special subalgebra handling for Cartesian product EJAs.

The matrix representation of an element in a Cartesian product EJA
will be an ordered tupe of matrices. When we move into a subalgebra,
the same element (considered as an element of the subalgebra) no
longer lives in a "Cartesian product EJA," since we give the
subalgebra its own basis. However, if we ever want to convert back,
then we need to know that the element originally came from a Cartesian
product EJA, because scalar fucking scaling doesn't fucking work on
ordered pairs!