Access integrand values in erpa-AC

[gabrielasd]

I needed to get the values of the $\alpha$ dependent integrand in ph-AC and to do so ended up modifying the structure of the erpa classmethod in the excitation module.
However, these changes broke the vectorization that function had and are potentially the cause of current inefficiency in the evaluation of ph-AC.

@msricher could you suggest another/nicer way to introduce this feature that is maybe closer to the original structure?

This is the commit for the changes 5f07062
Though, I think the original structure of the code is seen more clearly here:
https://github.com/gabrielasd/eomee/blob/939571c115e3acfd7d8e493a101dc77c9cf9a5f6/eomee/excitation.py#L89

In case it helps, the equation being implemented corresponds to the last box here:
https://github.com/gabrielasd/eomee/blob/refactor_ph_ac/docs/source/erpa.rst
(notice I have mislabelled the particle-hole equation as particle-particle.)