Rename dynamics solver functions

[cerisola]

Currently, all dynamics functions are called diffeqsolver, which, while to the point of what it does, might not be the clearest name for a new user. Furthermore, it makes the spin vs harmonic-oscillator difference very subtle (maybe this is good?).

We could rename these functions, so that they are more explicit, e.g. spin_dynamics(...) and harmonic_osc_dynamics(...) (or something like stochastic_dynamics(...) if we want to keep the same name for both).

To keep backwards compatibility, we could still define the diffeqsolver function names, so if current users update the library, their code keeps running (once we have proper releases and versioning, we can then drop the backwards compatibility at an appropriate major release).

[mekise]

I think we can keep this on hold until the major release with Chebyshev poly is released. Then, we can rename as spin_dynamics and harmonic_oscillator_dynamics dropping backward compatibility.