eja: handle tuples in parent algebras rather than in subclasses.

This is "necessary" because we won't always have a Cartesian product
algebra when our basis consists of tuples. Particularly in
element-subalgebras of Cartesian product algebras. It leads to more
special-casing, but whatever. Someday SageMath will know that both
matrix spaces and Cartesian products of vector spaces are themselves
vector spaces.