Package the analytics simplifications

Investigating #13 (closed) we have observed that we can eliminate H_0 from all numerators and obtain somewhat shorter expressions, although the downside is that don't yet know how to make this in \mathcal{O}(n) operations. Still, especially for analytics shorter formulas are nice. While the search for the general algorithm is still pending (#13 (closed)), if we have a clean and extensible API in #19 (closed), we can include the simplified expressions with relatively low cost. For that we would:

• Implement the analytic simplifications like we did before. The BlockOperatorSeries abstraction allows to do this rather nicely.
• Allow the user substitute their specific expressions for H and divide_by_energies, we'd first compute the simplified expressions internally, and then substitute the user-provided expressions as a follow-up step.

Blocked by #19 (closed), a partial solution of #13 (closed), probably belongs to %Package release, although can go either way.