eja: introduce an intermediate class for "concrete" algebras.

eja: introduce an intermediate class for "concrete" algebras.

There were two TODO items that are basically impossible: we can't
construct a "random" EJA from (say) an associative matrix algebra,
mainly because we don't know its rank. For that reason, it doesn't
make sense to have random_instance() defined in the parent class.
Now there's a subclass (KnownRankEJA) with those methods.