DirectSumEJA.
6. Switch to QQ in *all* algebras for _charpoly_coefficients().
+ This only works when we know that the basis can be rationalized...
+ which is the case at least for the concrete EJAs we provide,
+ but not in general.
7. Pass already_echelonized (default: False) and echelon_basis
(default: None) into the subalgebra constructor. The value of
some time, and usinf e.g. FreeModule_submodule_with_basis_field
we may somehow be able to pass the echelon basis straight in to
save time.
+
+ This may require supporting "basis" as a list of basis vectors
+ (as opposed to superalgebra elements) in the subalgebra constructor.