Make relationship between signals and hamiltonian_operators explicit in tutorial.

[hodgestar]

In https://qiskit.org/ecosystem/dynamics/tutorials/Rabi_oscillations.html the relationship between the hamiltonian_operators, the signals passed to the solver and the time-dependent term in the Hamiltonian isn't explicitly stated. I think readers who are not familiar with how such solvers structure things might not easily pick this up.

I also find the name hamiltonian_operators for the time-dependent / signal operators a bit strange. The static part of the Hamiltonian is also a Hamiltonian operator. Perhaps naming the argument signal_operators might this clearer?

[DanPuzzuoli]

I agree the naming is a little odd. It's been a while so I can't remember the alternatives we had considered, but I think we settled on the current naming conventions as a compromise between clarity and wanting to avoid the argument names being too long. (E.g. signal_operators isn't quite correct either as there are signals that can be supplied as coefficients for dissipator terms as well.)

At the stage I'm resistant to changing them as they've been with the package for so long. However, I think giving more explanation in the tutorial would be good. I've also just reread the Solver class documentation and that can be clarified as well.