eja: fix the natural representation in trivial subalgebras.

eja: fix the natural representation in trivial subalgebras.

The natural representation relies on knowing a matrix space, and in a
trivial subalgebra there ain't no matrices to have no spaces. To work
around that, the space is now computed/stored separately, in a new
natural_basis_space() method. This is then overridden in the subalgebra
class to do the right thing.