eja: replace the Hom stuff with a custom EJA operator class.

eja: replace the Hom stuff with a custom EJA operator class.

Implementing homsets as linear operators was always a hack, since
linear operators will in general not be algebra homomorphisms. We
wound up re-implementing addition, subtraction, etc. of operators
anyway, so in hindsight, there is no extra difficulty in creating our
own EJA operator class as a subclass of VectorSpaceMorphism.

This commit throws out the EJA morphism stuff, and replaces it with
an equivalent EJA operator class to remain mathematically sound.